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

[jhb]

Summary

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.

Test Plan

• wrote a little test program that used kvm_read and mmap to read osreldate. kvm_read still works after this change (and before it osreldate was the same via both means, but osreldate is in a "stable" page). mmap of /dev/kmem fails after this change (tested on amd64)

[kib]

sys/amd64/amd64/mem.c

177

You can reduce excessive indent by doing

{
  if (dev2unit(dev) != CDEV_MINOR_MEM || offset >= (1ULL << cpu_maxphyaddr))
    return (-1);
  *paddr = offset;
  return (0);
}

[jhb]

sys/amd64/amd64/mem.c

177

I have another pending change to this code (fixing the maxphyaddr on i386) so I'll leave this alone for now but perhaps fix it up there.

[jhb]

Forgot to refer to differential in commit, but this was committed in rS307332