f2fs: should avoid recursive filesystem ops

We need to use GFP_NOFS, since we did f2fs_lock_op().
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -1191,13 +1191,13 @@ static int __exchange_data_block(struct inode *src_inode,
 
 		src_blkaddr = f2fs_kvzalloc(F2FS_I_SB(src_inode),
 					array_size(olen, sizeof(block_t)),
-					GFP_KERNEL);
+					GFP_NOFS);
 		if (!src_blkaddr)
 			return -ENOMEM;
 
 		do_replace = f2fs_kvzalloc(F2FS_I_SB(src_inode),
 					array_size(olen, sizeof(int)),
-					GFP_KERNEL);
+					GFP_NOFS);
 		if (!do_replace) {
 			kvfree(src_blkaddr);
 			return -ENOMEM;