xfs: xfs_attr_set_iter() does not need to return EAGAIN

Now that the full xfs_attr_set_iter() state machine always
terminates with either the state being XFS_DAS_DONE on success or
an error on failure, we can get rid of the need for it to return
-EAGAIN whenever it needs to roll the transaction before running
the next state.

That is, we don't need to spray -EAGAIN return states everywhere,
the caller just check the state machine state for completion to
determine what action should be taken next. This greatly simplifies
the code within the state machine implementation as it now only has
to handle 0 for success or -errno for error and it doesn't need to
tell the caller to retry.

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>