xfs: introduce attr remove initial states into xfs_attr_set_iter

We need to merge the add and remove code paths to enable safe
recovery of replace operations. Hoist the initial remove states from
xfs_attr_remove_iter into xfs_attr_set_iter. We will make use of
them in the next patches.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison <Henderson&lt;allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Dave Chinner <david@fromorbit.com>