xfs: clean up xfs_btree_del_cursor callers

Less trivial cleanups of the error argument to xfs_btree_del_cursor;
these require some minor code refactoring.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>