Btrfs: get rid of sparse warnings

make C=2 fs/btrfs/ CF=-D__CHECK_ENDIAN__

I tried to filter out the warnings for which patches have already
been sent to the mailing list, pending for inclusion in btrfs-next.

All these changes should be obviously safe.

Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
diff --git a/fs/btrfs/check-integrity.c b/fs/btrfs/check-integrity.c
index 1f706e9..07f39a3 100644
--- a/fs/btrfs/check-integrity.c
+++ b/fs/btrfs/check-integrity.c
@@ -2441,7 +2441,7 @@
 		int num_copies;
 		int mirror_num;
 		const char *additional_string = NULL;
-		struct btrfs_disk_key tmp_disk_key;
+		struct btrfs_disk_key tmp_disk_key = {0};
 
 		btrfs_set_disk_key_objectid(&tmp_disk_key,
 					    BTRFS_ROOT_ITEM_KEY);