selftests/mm: add simple VM_PFNMAP tests based on mmap'ing /dev/mem

Let's test some basic functionality using /dev/mem.  These tests will
implicitly cover some PAT (Page Attribute Handling) handling on x86.

These tests will only run when /dev/mem access to the first two pages in
physical address space is possible and allowed; otherwise, the tests are
skipped.

On current x86-64 with PAT inside a VM, all tests pass:

	TAP version 13
	1..6
	# Starting 6 tests from 1 test cases.
	#  RUN           pfnmap.madvise_disallowed ...
	#            OK  pfnmap.madvise_disallowed
	ok 1 pfnmap.madvise_disallowed
	#  RUN           pfnmap.munmap_split ...
	#            OK  pfnmap.munmap_split
	ok 2 pfnmap.munmap_split
	#  RUN           pfnmap.mremap_fixed ...
	#            OK  pfnmap.mremap_fixed
	ok 3 pfnmap.mremap_fixed
	#  RUN           pfnmap.mremap_shrink ...
	#            OK  pfnmap.mremap_shrink
	ok 4 pfnmap.mremap_shrink
	#  RUN           pfnmap.mremap_expand ...
	#            OK  pfnmap.mremap_expand
	ok 5 pfnmap.mremap_expand
	#  RUN           pfnmap.fork ...
	#            OK  pfnmap.fork
	ok 6 pfnmap.fork
	# PASSED: 6 / 6 tests passed.
	# Totals: pass:6 fail:0 xfail:0 xpass:0 skip:0 error:0

However, we are able to trigger:

[   27.888251] x86/PAT: pfnmap:1790 freeing invalid memtype [mem 0x00000000-0x00000fff]

There are probably more things worth testing in the future, such as
MAP_PRIVATE handling.  But this set of tests is sufficient to cover most
of the things we will rework regarding PAT handling.

Link: https://lkml.kernel.org/r/20250509153033.952746-1-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Dev Jain <dev.jain@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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