f2fs: factor out an f2fs_default_check function

The current options parsing function both parses options and validates
them - factor the validation out to reduce the size of the function and
make transition to the new mount API possible, because under the new mount
API, options are parsed one at a time, and cannot all be tested at the end
of the parsing function.

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