btrfs: reflink: initialize return value to 0 in btrfs_extent_same()

Fix a warning reported by smatch that ret could be returned without
initialized.  The dedupe operations are supposed to to return 0 for a 0
length range but the caller does not pass olen == 0. To keep this
behaviour and also fix the warning initialize ret to 0.
Reviewed-by: NFilipe Manana <fdmanana@suse.com>
Signed-off-by: NSidong Yang <realwakka@gmail.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/reflink.c

@@ -649,7 +649,7 @@ static int btrfs_extent_same_range(struct inode *src, u64 loff, u64 len,
 static int btrfs_extent_same(struct inode *src, u64 loff, u64 olen,
 			     struct inode *dst, u64 dst_loff)
 {
-	int ret;
+	int ret = 0;
 	u64 i, tail_len, chunk_count;
 	struct btrfs_root *root_dst = BTRFS_I(dst)->root;