UPSTREAM: 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.)

Change-Id: Idc4378349bcaf393b41615837e3494e3ed98983e
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit 9cca4987)