kern linker: Do not unload a module if it has dependants

Despite the name, linker_file_unload() will drop a reference and return
success when the module file has dependants, i.e. it has more than one
reference. When user request to unload such modules then the kernel
should reject unambiguously and immediately.

PR: 274986
Reviewed by: dfr, dab, jhb
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D42527