linker: Reset DMAP protections in link_elf_unload_file()

On x86, when a preloaded kernel module is unloaded, we free the backing
(physically contiguous) pages. The ET_REL linker will have adjusted
protections on segments of the preloaded file, which updates the direct
map, so the original protections must be restored when unloading the
module.

Previously this was handled in kmem_bootstrap_free(), but there is no
apparent reason not to handle this within the kernel linker. Moreover,
we were not resetting permissions in the kernel map on arm64.

Reviewed by: alc, kib
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D54438