btrfs: split out the mount option validation code into its own helper

We're going to need to validate mount options after they're all parsed
with the new mount API, split this code out into its own helper so we
can use it when we swap over to the new mount API.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ minor adjustments in the messages ]
Signed-off-by: David Sterba <dsterba@suse.com>