spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

ext2fs: extract crc16 into sys/crc16.h

deduplicate this as it might be needed for other drivers (e.g. Apple SPI-HID)

Sponsored by: https://www.patreon.com/valpackett
Reviewed by: chuck, imp
MFC after: 1 month
Differential revision: https://reviews.freebsd.org/D32879