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
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34264