xfs: fix a UAF problem in xattr repair

xfs: fix a UAF problem in xattr repair

The xchk_setup_xattr_buf function can allocate a new value buffer, which
means that any reference to ab->value before the call could become a
dangling pointer.  Fix this by moving an assignment to after the buffer
setup.

Cc: stable@vger.kernel.org # v6.10
Fixes: e47dcf113ae348 ("xfs: repair extended attributes")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>