powerpc/powernv/memtrace: Allow mmaping trace buffers

Let the memory removed from the linear mapping to be used for the trace
buffers be mmaped. This is a useful way of providing cache-inhibited
memory for the alignment_handler selftest.

Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
[mpe: make memtrace_mmap() static as noticed by lkp@intel.com]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210225032108.1458352-1-jniethe5@gmail.com