msdosfs: Fix mounting when the device sector size is >512B

HugeSectors * BytesPerSec should be computed before converting
HugeSectors to a DEV_BSIZE-based count.

Fixes: ba2c98389b78 ("msdosfs: sanity check sector count from BPB")
Reviewed by: kib
Sponsored by: The FreeBSD Foundation

(cherry picked from commit c7cd607a4e28233ab6679ee330c0a4836414bb0a)