kern linker: Do not retry loading modules on EEXIST

[zlei]

Summary

LINKER_LOAD_FILE() will call linker_load_dependencies() which would
return EEXIST in case the module has been already compiled into the
kernel. Since we now recognize the module class then there is no need
to retry loading with a different linker, otherwise userland will get
a misleading error ENOEXEC.

PR: 274936
MFC after: 2 weeks

Test Plan

Load modules that are compiled into kernel, verify the output of kldload.

# kldload virtio.ko
kldload: can't load virtio.ko: module already loaded or in kernel

Or

# kldload -n virtio.ko && echo "virtio.ko loaded"
virtio.ko loaded

[dfr]

Looks good to me.