dm exception store: fix exstore lookup to be case insensitive

When snapshots are created using 'p' instead of 'P' as the
exception store type, the device-mapper table loading fails.

This patch makes the code case insensitive as intended and fixes some
regressions reported with device-mapper snapshots.
Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-exception-store.c

@@ -216,7 +216,7 @@ int dm_exception_store_create(struct dm_target *ti, int argc, char **argv,
 		return -EINVAL;
 	}
 
-	type = get_type(argv[1]);
+	type = get_type(&persistent);
 	if (!type) {
 		ti->error = "Exception store type not recognised";
 		r = -EINVAL;