xfs: refactor extended attribute list operation

When we're iterating the attribute list and we can't find our previous
location based off the attribute cursor, we'll instead walk down the
attribute btree from the root trying to find where we left off.  Move
this code into a separate function for later cleanups.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>