ktls: catch invalid parameters earlier

Move safety checks forward from ktls_session_create() to
ktls_copyin_tls_enable(). Prevents zero mallocs, and excessively
large kernel mallocs.

Reported-by: syzbot+72022fa9163fa958b66c@syzkaller.appspotmail.com
Reported-by: syzbot+8992893e13058ce0670a@syzkaller.appspotmail.com
Sponsored by: NetApp, Inc.
X-NetApp-PR: #79
Reviewed By: tuexen
Differential Revision: https://reviews.freebsd.org/D44364