Btrfs: fix compiler warning in file.c

While compiling Btrfs, I got following messages:

  CC [M]  fs/btrfs/file.o
fs/btrfs/file.c: In function '__btrfs_buffered_write':
fs/btrfs/file.c:909: warning: 'ret' may be used uninitialized in this function
  CC [M]  fs/btrfs/tree-defrag.o

This patch fixes compiler warning.
Signed-off-by: NTsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/file.c

@@ -906,7 +906,7 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
 	unsigned long last_index;
 	size_t num_written = 0;
 	int nrptrs;
-	int ret;
+	int ret = 0;
 
 	nrptrs = min((iov_iter_count(i) + PAGE_CACHE_SIZE - 1) /
 		     PAGE_CACHE_SIZE, PAGE_CACHE_SIZE /