crypto: Permit variable-sized IVs for ciphers with a reinit hook.

Add a 'len' argument to the reinit hook in 'struct enc_xform' to
permit support for AEAD ciphers such as AES-CCM and Chacha20-Poly1305
which support different nonce lengths.

Reviewed by: markj
Sponsored by: Chelsio Communications, The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32105