Btrfs: fix use of uninitialized err variable

fs/btrfs/file.c: In function ‘prepare_pages.isra.18’:
fs/btrfs/file.c:1265:6: warning: ‘err’ may be used uninitialized in this function [-Wuninitialized]
Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/file.c

@@ -1244,7 +1244,7 @@ static noinline int prepare_pages(struct inode *inode, struct page **pages,
 	int i;
 	unsigned long index = pos >> PAGE_CACHE_SHIFT;
 	gfp_t mask = btrfs_alloc_write_mask(inode->i_mapping);
-	int err;
+	int err = 0;
 	int faili;
 
 	for (i = 0; i < num_pages; i++) {