f2fs: relax migratepage for atomic written page

In order to avoid lock contention for atomic written pages, we'd better give
EBUSY in f2fs_migrate_page when mode is asynchronous. We expect it will be
released soon as transaction commits.
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -2203,8 +2203,12 @@ int f2fs_migrate_page(struct address_space *mapping,
 	BUG_ON(PageWriteback(page));
 
 	/* migrating an atomic written page is safe with the inmem_lock hold */
-	if (atomic_written && !mutex_trylock(&fi->inmem_lock))
-		return -EAGAIN;
+	if (atomic_written) {
+		if (mode != MIGRATE_SYNC)
+			return -EBUSY;
+		if (!mutex_trylock(&fi->inmem_lock))
+			return -EAGAIN;
+	}
 
 	/*
 	 * A reference is expected if PagePrivate set when move mapping,