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.

---
 fs/btrfs/file.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff -urNp linux-2.6.39-rc1/fs/btrfs/file.c linux-2.6.39-rc1.new/fs/btrfs/file.c
@@ -906,7 +906,7 @@ static noinline ssize_t __btrfs_buffered
 	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 /