xfs: Refactor xfs_attr_rmtval_remove

Refactor xfs_attr_rmtval_remove to add helper function
__xfs_attr_rmtval_remove. We will use this later when we introduce
delayed attributes.  This function will eventually replace
xfs_attr_rmtval_remove

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