fusefs: standardize on OPNOTSUPP for posix_fallocate(2)

POSIX Issue 7 had allowed EINVAL for this case, but issue 8 moves it
to ENOTSUP instead. ZFS uses the latter and we have some software in
ports already that's wanting to use that to detect the filesystem not
supporting it, so let's standardize on it.

Reviewed by: imp (previous version), asomers, kib
Differential Revision: https://reviews.freebsd.org/D53535