fsck.f2fs: open ro disk if we want to check fs only

fsck.f2fs: open ro disk if we want to check fs only

This patch fixes the "open failure" issue on ro disk, reported by Hagbard.

"
 If I boot with kernel option "ro rootfstype=f2fs
 I get the following halfway trough boot:

  * Checking local filesystems  ...
 Info: Use default preen mode
 Info: Mounted device!
 Info: Check FS only due to RO
         Error: Failed to open the device!
  * Filesystems couldn't be fixed
"

Reported-by: Hagbard Celine <hagbardcelin@gmail.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>