Don't invoke semunload() if seminit() fails during MOD_LOAD.

Don't invoke semunload() if seminit() fails during MOD_LOAD.

The module handler code invokes a MOD_UNLOAD event immediately if
MOD_LOAD fails.  The result was that if seminit() failed, semunload()
was invoked twice.  semunload() is not idempotent however and would
try to remove it's process_exit eventhandler twice resulting in a
panic.

Reviewed by:	kib, markj
Obtained from:	CheriBSD
MFC after:	1 month
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D26696