Remove the cloned file descriptors for /dev/crypto.

Crypto file descriptors were added in the original OCF import as a way
to provide per-open data (specifically the list of symmetric
sessions). However, this gives a bit of a confusing API where one has
to open /dev/crypto and then invoke an ioctl to obtain a second file
descriptor. This also does not match the API used with /dev/crypto on
other BSDs or with Linux's /dev/crypto driver.

Character devices have gained support for per-open data via cdevpriv
since OCF was imported, so use cdevpriv to simplify the userland API
by permitting ioctls directly on /dev/crypto descriptors.

To provide backwards compatibility, CRIOGET now opens another
/dev/crypto descriptor via kern_openat() rather than dup'ing the
existing file descriptor. This preserves prior semantics in case
CRIOGET is invoked multiple times on a single file descriptor.

Reviewed by: markj
Relnotes: yes
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D27302