f2fs: pass sbi rather than sb to parse_options()

With the new mount API the sb will not be available during initial option
parsing, which will happen before fill_super reads sb from disk.

Now that the sb is no longer directly referenced in parse_options, switch
it to use sbi.

(Note that all calls to f2fs_sb_has_* originating from parse_options will
need to be deferred to later before we can use the new mount API.)

Signed-off-by: Eric Sandeen <sandeen@redhat.com>