Fix page fault that occurred when trying to initialize preloaded kernel module,

Fix page fault that occurred when trying to initialize preloaded kernel module,
the dependency of which was preloaded, but failed to initialize.  Previously,
kernel dereferenced NULL pointer returned by modlist_lookup2(); now, when this
happens, we unload the dependent module.  Since the depended_files list is
sorted in dependency order, this properly propagates, unloading modules that
depend on failed ones.

From the user point of view, this prevents the kernel from panicing when
trying to boot kernel compiled without KDTRACE_HOOKS with dtraceall_load="YES"
in /boot/loader.conf.

Reviewed by:	kib