xfs: Factor out new helper functions xfs_attr_rmtval_set

Break xfs_attr_rmtval_set into two helper functions
xfs_attr_rmt_find_hole and xfs_attr_rmtval_set_value.
xfs_attr_rmtval_set rolls the transaction between the helpers, but
delayed operations cannot.  We will use the helpers later when
constructing new delayed attribute routines.

Signed-off-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Chandan Rajendra <chandanrlinux@gmail.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Acked-by: Dave Chinner <dchinner@redhat.com>