MFC r325422: posix_fallocate.2: add an EINVAL errno case

As of r325320 in HEAD posix_fallocate returns EINVAL on ZFS to indicate
that the underlying filesystem does not support this operation, per
POSIX.1-2008. Document this case in the man page.

Note that r325320 has not yet been merged to stable/11, and may or may
not be. However, we should document that EINVAL may be returned if the
filesystem does not support posix_fallocate (even if we don't actually
do so in stable/11), as software should be prepared to handle that case.

Discussed with: avg
Sponsored by: The FreeBSD Foundation