ktls: catch invalid parameters earlier

[rscheff]

Summary

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

Reported-by: [e-mail]
Sponsored by: NetApp, Inc.
X-NetApp-PR: #79

[rscheff]

• some compiler trickery to the correct struct member size