posix_fallocate(2): move unsupported case to EOPNOTSUPP

ZFS has since been changed to use EOPNOTSUPP instead of EINVAL, and
fusefs/nfs are following suit. POSIX Issue 8 had also made this move,
so it makes sense for us to standardize on EOPNOTSUPP.

Note in the HISTORY section where we're diverging from our previous
versions to align with the new standard.

Reviewed by: asomers, imp (both previous version), kib
Differential Revision: https://reviews.freebsd.org/D53537