mm: mm_struct: remove 16 bytes of alignment padding on 64 bit builds

Reorder mm_struct to remove 16 bytes of alignment padding on 64 bit
builds. On my config this shrinks mm_struct by enough to fit in one
fewer cache lines and allows more objects per slab in mm_struct
kmem_cache under SLUB.

slabinfo before patch :-
Sizes (bytes) Slabs
--------------------------------
Object : 848 Total : 9
SlabObj: 896 Full : 2
SlabSiz: 16384 Partial: 5
Loss : 48 CpuSlab: 2
Align : 64 Objects: 18

slabinfo after :-
Sizes (bytes) Slabs
--------------------------------
Object : 832 Total : 7
SlabObj: 832 Full : 2
SlabSiz: 16384 Partial: 3
Loss : 0 CpuSlab: 2
Align : 64 Objects: 19

Signed-off-by: Richard Kennedy <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/mm_types.h

···
 	atomic_t mm_users;			/* How many users with user space? */
 	atomic_t mm_count;			/* How many references to "struct mm_struct" (users count as 1) */
 	int map_count;				/* number of VMAs */
-	struct rw_semaphore mmap_sem;
+
 	spinlock_t page_table_lock;		/* Protects page tables and some counters */
+	struct rw_semaphore mmap_sem;
 
 	struct list_head mmlist;		/* List of maybe swapped mm's.	These are globally strung
 						 * together off init_mm.mmlist, and are protected
···
 	unsigned int token_priority;
 	unsigned int last_interval;
 
+	/* How many tasks sharing this mm are OOM_DISABLE */
+	atomic_t oom_disable_count;
+
 	unsigned long flags; /* Must use atomic bitops to access the bits */
 
 	struct core_state *core_state; /* coredumping support */
···
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 	pgtable_t pmd_huge_pte; /* protected by page_table_lock */
 #endif
-	/* How many tasks sharing this mm are OOM_DISABLE */
-	atomic_t oom_disable_count;
 };
 
 /* Future-safe accessor for struct mm_struct's cpu_vm_mask. */