Invoke the kld_* event handlers from linker_load_file() and

Invoke the kld_* event handlers from linker_load_file() and
linker_unload_file() rather than kern_kldload() and kern_kldunload(). This
ensures that the handlers are invoked for files that are loaded/unloaded
automatically as dependencies. Previously, they were only invoked for files
loaded by a user.

As a side effect, the kld_load and kld_unload handlers are now invoked with
the kernel linker lock exclusively held.

Reported by:	avg
Reviewed by:	jhb
MFC after:	2 weeks