kern linker: Do not touch userrefs of the kernel file

A nonzero userrefs of a linker file indicates that the file, either
loaded from kldload(2) or preloaded, can be unloaded via kldunload(2).
As for the kernel file, it can be unloaded by the loader but should not
be after initialization.

This change fixes regression from d9ce8a41eac9 which incidentally
increases userrefs of the kernel file.

Reviewed by: dfr, dab, jhb
Fixes: d9ce8a41eac9 kern_linker: Handle module-loading failures in preloaded .ko files
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D42530

(cherry picked from commit 39450eba8e6cd5b83def724481127b58ecc1fcc1)
(cherry picked from commit 7da45efb0921e9937353992226f34b25ae197545)