xfs: Fix default ASSERT in xfs_attr_set_iter

This ASSERT checks for the state value of RM_SHRINK in the set path
which should never happen.  Change to ASSERT(0);

Suggested-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>