Drop support for using mmap() with /dev/kmem.

Using the device pager with /dev/kmem is not stable since KVA mappings
are transient, but the device pager caches the PA associated with a
given offset forever. Interestingly, mips' implementation of
memmap() already refused requests for /dev/kmem.

Note that kvm_read/kvm_write do not use mmap, but use read and write on
/dev/kmem, so this should not affect libkvm users.

Reviewed by: kib
MFC after: 2 months