f2fs: handle readonly filesystem in f2fs_ioc_shutdown()

If mountpoint is readonly, we should allow shutdowning filesystem
successfully, this fixes issue found by generic/599 testcase of
xfstest.
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -2232,8 +2232,15 @@ static int f2fs_ioc_shutdown(struct file *filp, unsigned long arg)
 
 	if (in != F2FS_GOING_DOWN_FULLSYNC) {
 		ret = mnt_want_write_file(filp);
-		if (ret)
+		if (ret) {
+			if (ret == -EROFS) {
+				ret = 0;
+				f2fs_stop_checkpoint(sbi, false);
+				set_sbi_flag(sbi, SBI_IS_SHUTDOWN);
+				trace_f2fs_shutdown(sbi, in, ret);
+			}
 			return ret;
+		}
 	}
 
 	switch (in) {