linker: Don't invoke dtors without having invoked ctors

[markj]

Summary

I have a kernel module which fails to load because of an unrecognized
relocation type. link_elf_load_file() fails before the module's ctors
are invoked and it calls linker_file_unload(), which causes the module's
dtors to be executed, resulting in a kernel panic.

Add a flag to the linker file to ensure that dtors are not invoked if
unloading due to an error.

At the moment I only implemented this for link_elf_obj.c since
link_elf.c doesn't invoke dtors, but I refactored link_elf.c to make
them more similar.

Fixes: 9e575fadf491 ("link_elf_obj: Invoke fini callbacks")

[kib]

sys/kern/link_elf_obj.c

664

In this form the assert can be MPASS. For KASSERT, at least the linker file name would be useful to print.

sys/sys/linker.h

85

Might be, change the bool to 'epoch' int. Indicating no ctr called/ctr called/dtr called. Then you can assert e.g. that dtr not called twice.

[markj]

sys/sys/linker.h

85

I made it into an enum.

[markj]

Apply kib's suggestions.

[zlei]

Looks good to me.