dm thin: clean up compiler warning

Clean up "warning: dubious: !x & y".  Also make it clear that
__snapshotted_since() returns a bool and that dm_thin_lookup_result's
'shared' member is a flag.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
diff --git a/drivers/md/dm-thin-metadata.h b/drivers/md/dm-thin-metadata.h
index b88918c..7b47c0a 100644
--- a/drivers/md/dm-thin-metadata.h
+++ b/drivers/md/dm-thin-metadata.h
@@ -119,7 +119,7 @@
 
 struct dm_thin_lookup_result {
 	dm_block_t block;
-	int shared;
+	unsigned shared:1;
 };
 
 /*