fusefs: Fix iosize for FUSE_WRITE in 7.8 compat mode

When communicating with a FUSE server that implements version 7.8 (or older)
of the FUSE protocol, the FUSE_WRITE request structure is 16 bytes shorter
than normal. The protocol version check wasn't applied universally, leading
to an extra 16 bytes being sent to such servers. The extra bytes were
allocated and bzero()d, so there was no information disclosure.

Reviewed by: emaste
MFC after: 3 days
MFC-With: r350665
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21557