ktls: Don't leak ktls session objects for certain errors.

ktls_cleanup() does not free ktls session objects, it merely
cleans (and frees) members of the object.

Change callers to use ktls_free() instead.

Reviewed by: gallatin, markj
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D37352

(cherry picked from commit d01db2b83705eb3557a32ea13143997cdb8135a3)