f2fs: defer readonly check vs norecovery

Defer the readonly-vs-norecovery check until after option parsing is done
so that option parsing does not require an active superblock for the test.
Add a helpful message, while we're at it.

(I think could be moved back into parsing after we switch to the new mount
API if desired, as the fs context will have RO state available.)

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>