hpilo: Replace one-element array with flexible-array member

There is a regular need in the kernel to provide a way to declare
having a dynamically sized set of trailing elements in a structure.
Kernel code should always use “flexible array members”[1] for these
cases. The older style of one-element or zero-length arrays should
no longer be used[2].

For this particular case, it is important to notice that the cachelines
change from 7 to 6 after the flexible-array conversion:

$ pahole -C 'fifo' drivers/misc/hpilo.o
struct fifo {
u64 nrents; /* 0 8 */
u64 imask; /* 8 8 */
u64 merge; /* 16 8 */
u64 reset; /* 24 8 */
u8 pad_0[96]; /* 32 96 */
/* --- cacheline 2 boundary (128 bytes) --- */
u64 head; /* 128 8 */
u8 pad_1[120]; /* 136 120 */
/* --- cacheline 4 boundary (256 bytes) --- */
u64 tail; /* 256 8 */
u8 pad_2[120]; /* 264 120 */
/* --- cacheline 6 boundary (384 bytes) --- */
u64 fifobar[1]; /* 384 8 */

/* size: 392, cachelines: 7, members: 10 */
/* last cacheline: 8 bytes */
};

$ pahole -C 'fifo' drivers/misc/hpilo.o
struct fifo {
u64 nrents; /* 0 8 */
u64 imask; /* 8 8 */
u64 merge; /* 16 8 */
u64 reset; /* 24 8 */
u8 pad_0[96]; /* 32 96 */
/* --- cacheline 2 boundary (128 bytes) --- */
u64 head; /* 128 8 */
u8 pad_1[120]; /* 136 120 */
/* --- cacheline 4 boundary (256 bytes) --- */
u64 tail; /* 256 8 */
u8 pad_2[120]; /* 264 120 */
/* --- cacheline 6 boundary (384 bytes) --- */
u64 fifobar[]; /* 384 0 */

/* size: 384, cachelines: 6, members: 10 */
};

Lastly, remove unnecessary parentheses in fifo_sz() and fix the following
checkpatch.pl warning for the whole fifo structure:

WARNING: please, no spaces at the start of a line

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://github.com/KSPP/linux/issues/79

Tested-by: kernel test robot <[e-mail]>
Link: https://github.com/GustavoARSilva/linux-hardening/blob/master/cii/kernel-ci/hpilo-20200714.md
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Link: https://lore.kernel.org/r/20200714154449.GA26153@embeddedor
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/misc/hpilo.c

···
 static inline int fifo_sz(int nr_entry)
 {
 	/* size of a fifo is determined by the number of entries it contains */
-	return (nr_entry * sizeof(u64)) + FIFOHANDLESIZE;
+	return nr_entry * sizeof(u64) + FIFOHANDLESIZE;
 }
 
 static void fifo_setup(void *base_addr, int nr_entry)

drivers/misc/hpilo.h

···
 #define ILO_START_ALIGN	4096
 #define ILO_CACHE_SZ 	 128
 struct fifo {
-    u64 nrents;	/* user requested number of fifo entries */
-    u64 imask;  /* mask to extract valid fifo index */
-    u64 merge;	/*  O/C bits to merge in during enqueue operation */
-    u64 reset;	/* set to non-zero when the target device resets */
-    u8  pad_0[ILO_CACHE_SZ - (sizeof(u64) * 4)];
+	u64 nrents;	/* user requested number of fifo entries */
+	u64 imask;  /* mask to extract valid fifo index */
+	u64 merge;	/*  O/C bits to merge in during enqueue operation */
+	u64 reset;	/* set to non-zero when the target device resets */
+	u8  pad_0[ILO_CACHE_SZ - (sizeof(u64) * 4)];
 
-    u64 head;
-    u8  pad_1[ILO_CACHE_SZ - (sizeof(u64))];
+	u64 head;
+	u8  pad_1[ILO_CACHE_SZ - (sizeof(u64))];
 
-    u64 tail;
-    u8  pad_2[ILO_CACHE_SZ - (sizeof(u64))];
+	u64 tail;
+	u8  pad_2[ILO_CACHE_SZ - (sizeof(u64))];
 
-    u64 fifobar[1];
+	u64 fifobar[];
 };
 
 /* convert between struct fifo, and the fifobar, which is saved in the ccb */
-#define FIFOHANDLESIZE (sizeof(struct fifo) - sizeof(u64))
+#define FIFOHANDLESIZE (sizeof(struct fifo))
 #define FIFOBARTOHANDLE(_fifo) \
 	((struct fifo *)(((char *)(_fifo)) - FIFOHANDLESIZE))