xfs: convert flex-array declarations in xfs attr shortform objects

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

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