Allow block cloning to be interrupted by a signal.

Even though block cloning is much faster than regular copying,
it is not instantaneous - the file might be large and the recordsize
small. It would be nice to be able to interrupt it with a signal
(e.g., SIGINFO on FreeBSD to see the progress).

Signed-off-by: Pawel Jakub Dawidek <pawel@dawidek.net>

module/zfs/zfs_vnops.c

@@ -1412,6 +1412,11 @@ zfs_clone_range(znode_t *inzp, uint64_t *inoffp, znode_t *outzp,
 		outoff += size;
 		len -= size;
 		done += size;
+
+		if (issig(JUSTLOOKING) && issig(FORREAL)) {
+			error = SET_ERROR(EINTR);
+			break;
+		}
 	}
 
 	vmem_free(bps, sizeof (bps[0]) * maxblocks);