xfs: convert flex-array declarations in struct xfs_attrlist*

As of 6.5-rc1, UBSAN trips over the attrlist ioctl definitions using an
array length of 1 to pretend to be a flex array.  Kernel compilers have
to support unbounded array declarations, so let's correct this.  This
may cause friction with userspace header declarations, but suck is life.

================================================================================
UBSAN: array-index-out-of-bounds in fs/xfs/xfs_ioctl.c:345:18
index 1 is out of range for type '__s32 [1]'
Call Trace:
 <TASK>
 dump_stack_lvl+0x33/0x50
 __ubsan_handle_out_of_bounds+0x9c/0xd0
 xfs_ioc_attr_put_listent+0x413/0x420 [xfs 4a986a89a77bb77402ab8a87a37da369ef6a3f09]
 xfs_attr_list_ilocked+0x170/0x850 [xfs 4a986a89a77bb77402ab8a87a37da369ef6a3f09]
 xfs_attr_list+0xb7/0x120 [xfs 4a986a89a77bb77402ab8a87a37da369ef6a3f09]
 xfs_ioc_attr_list+0x13b/0x2e0 [xfs 4a986a89a77bb77402ab8a87a37da369ef6a3f09]
 xfs_attrlist_by_handle+0xab/0x120 [xfs 4a986a89a77bb77402ab8a87a37da369ef6a3f09]
 xfs_file_ioctl+0x1ff/0x15e0 [xfs 4a986a89a77bb77402ab8a87a37da369ef6a3f09]
 vfs_ioctl+0x1f/0x60

The kernel and xfsprogs code that uses these structures will not have
problems, but the long tail of external user programs might.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kees Cook <keescook@chromium.org>

Author: Darrick J. Wong

fs/xfs/libxfs/xfs_fs.h

@@ -592,12 +592,12 @@ typedef struct xfs_attrlist_cursor {
 struct xfs_attrlist {
 	__s32	al_count;	/* number of entries in attrlist */
 	__s32	al_more;	/* T/F: more attrs (do call again) */
-	__s32	al_offset[1];	/* byte offsets of attrs [var-sized] */
+	__s32	al_offset[];	/* byte offsets of attrs [var-sized] */
 };
 
 struct xfs_attrlist_ent {	/* data from attr_list() */
 	__u32	a_valuelen;	/* number bytes in value of attr */
-	char	a_name[1];	/* attr name (NULL terminated) */
+	char	a_name[];	/* attr name (NULL terminated) */
 };
 
 typedef struct xfs_fsop_attrlist_handlereq {