fs: btrfs: Fix usage of uninitialized variables

The variable res should be initialized to 0 in these functions,
because if the searched key is not found, the variable is used
uninitialized.

Reported-by: Coverity (CID: 167335)
Reported-by: Coverity (CID: 167336)
Reported-by: Coverity (CID: 167337)
Signed-off-by: Marek Behun <marek.behun@nic.cz>
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 0d3da28..0af0427 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -324,7 +324,7 @@
 	struct btrfs_path path;
 	struct btrfs_key key;
 	struct btrfs_file_extent_item *extent;
-	int res;
+	int res = 0;
 	u64 rd, rd_all = -1ULL;
 
 	key.objectid = inr;