xfs: fix xfs_rmap_has_other_keys usage of ECANCELED

In e7ee96df, we converted all ITER_ABORT users to use ECANCELED
instead, but we forgot to teach xfs_rmap_has_other_keys not to return
that magic value to callers.  Fix it now by using ECANCELED both to
abort the iteration and to signal that we found another reverse mapping.
This enables us to drop the separate boolean flag.

Fixes: e7ee96df ("xfs: remove all *_ITER_ABORT values")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>