iflib: remove kobject class reference increment

Commit message from Jake:
In iflib_register, the context is initialized as a kobject using the
device driver's "driver" kobject class. As part of this, the function
mistakenly increments the ref counter.

The ref counter is incremented twice, once in the code directly, and
once again by kobj_class_compile. However, there is no associated
decrement in the detach path. Because of this, the ref counter will
never go back down to zero, and thus the kobject method table will never
be released.

Remove this unnecessary reference count increment.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>

Submitted by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed by: jhb@, erj@
MFC after: 3 days
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D21125