ocfs2: replace zero-length arrays with DECLARE_FLEX_ARRAY() helper

Zero-length arrays are deprecated and we are moving towards adopting C99
flexible-array members, instead. So, replace zero-length array
declarations in a couple of structures and unions with the new
DECLARE_FLEX_ARRAY() helper macro.

This helper allows for a flexible-array member in a union and as only
member in a structure.

Also, this addresses multiple warnings reported when building with
Clang-15 and -Wzero-length-array.

Lastly, this will also help memcpy (in a coming hardening update) execute
proper bounds-checking on variable length object i_symlink at
fs/ocfs2/namei.c:1973:

fs/ocfs2/namei.c:
1973 memcpy((char *) fe->id2.i_symlink, symname, l);

Link: https://github.com/KSPP/linux/issues/21
Link: https://github.com/KSPP/linux/issues/193
Link: https://github.com/KSPP/linux/issues/197
Link: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
Link: https://lkml.kernel.org/r/YxKY6O2hmdwNh8r8@work
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Reviewed-by: Joseph Qi <[e-mail]>
Cc: Mark Fasheh <[e-mail]>
Cc: Joel Becker <[e-mail]>
Cc: Junxiao Bi <[e-mail]>
Cc: Changwei Ge <[e-mail]>
Cc: Gang He <[e-mail]>
Cc: Jun Piao <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

fs/ocfs2/ocfs2_fs.h

···
  * value -1 (0xFFFF) is OCFS2_INVALID_SLOT.  This marks a slot empty.
  */
 struct ocfs2_slot_map {
-/*00*/	__le16 sm_slots[0];
+/*00*/	DECLARE_FLEX_ARRAY(__le16, sm_slots);
 /*
  * Actual on-disk size is one block.  OCFS2_MAX_SLOTS is 255,
  * 255 * sizeof(__le16) == 512B, within the 512B block minimum blocksize.
···
  * i_size.
  */
 struct ocfs2_slot_map_extended {
-/*00*/	struct ocfs2_extended_slot se_slots[0];
+/*00*/	DECLARE_FLEX_ARRAY(struct ocfs2_extended_slot, se_slots);
 /*
  * Actual size is i_size of the slot_map system file.  It should
  * match s_max_slots * sizeof(struct ocfs2_extended_slot)
···
 		struct ocfs2_extent_list	i_list;
 		struct ocfs2_truncate_log	i_dealloc;
 		struct ocfs2_inline_data	i_data;
-		__u8               		i_symlink[0];
+		DECLARE_FLEX_ARRAY(__u8,	i_symlink);
 	} id2;
 /* Actual on-disk size is one block */
 };
···
 /*30*/	struct ocfs2_block_check bg_check;	/* Error checking */
 	__le64   bg_reserved2;
 /*40*/	union {
-		__u8    bg_bitmap[0];
+		DECLARE_FLEX_ARRAY(__u8, bg_bitmap);
 		struct {
 			/*
 			 * Block groups may be discontiguous when