nandfs: avoid integer overflow in nandfs_get_dat_bdescs_ioctl

nandfs was removed in head in r349352 and in any case was not built by
default, but address the potential integer overflow in case someone does
enable it and manages to avoid a panic from other nandfs issues.

admbugs: 815
Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Reviewed by: imp
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21232