stand: module: unlink the entire tail when dependencies fail to load

Assume you have loader configured to load linux64, which has a
dependency on both linux_common and mqueuefs but neither the kernel
nor kernel config in question have the mqueuefs module included.

When the load command for linux64 fails to find mqueuefs, it will
free both linux64 and linux_common as they were loaded first, but only
linux64 gets removed from the module list. As a result, future
traversals hit an easy use-after-free with linux_common.

Fix it so that we unlink the entire tail of the list. Anything after
the initially loaded module is, by definition, a dependency on the
loaded module while we're still in the load command, so we can just
discard the entire tail. If linux_common were loaded before linux64, it
should not move to a position during this load where it would suddenly
be missing from the view presented to the kernel.

Reported by: philip
Reviewed by: imp, philip, tsoome
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D45731