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: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>