xfs: convert flex-array declarations in xfs attr leaf blocks

As of 6.5-rc1, UBSAN trips over the ondisk extended attribute leaf block
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.

================================================================================
UBSAN: array-index-out-of-bounds in fs/xfs/libxfs/xfs_attr_leaf.c:2535:24
index 2 is out of range for type '__u8 [1]'
Call Trace:
 <TASK>
 dump_stack_lvl+0x33/0x50
 __ubsan_handle_out_of_bounds+0x9c/0xd0
 xfs_attr3_leaf_getvalue+0x2ce/0x2e0 [xfs 4a986a89a77bb77402ab8a87a37da369ef6a3f09]
 xfs_attr_leaf_get+0x148/0x1c0 [xfs 4a986a89a77bb77402ab8a87a37da369ef6a3f09]
 xfs_attr_get_ilocked+0xae/0x110 [xfs 4a986a89a77bb77402ab8a87a37da369ef6a3f09]
 xfs_attr_get+0xee/0x150 [xfs 4a986a89a77bb77402ab8a87a37da369ef6a3f09]
 xfs_xattr_get+0x7d/0xc0 [xfs 4a986a89a77bb77402ab8a87a37da369ef6a3f09]
 __vfs_getxattr+0xa3/0x100
 vfs_getxattr+0x87/0x1d0
 do_getxattr+0x17a/0x220
 getxattr+0x89/0xf0

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