selftests: add a MREMAP_DONTUNMAP selftest for shmem

selftests: add a MREMAP_DONTUNMAP selftest for shmem

This test extends the current mremap tests to validate that the
MREMAP_DONTUNMAP operation can be performed on shmem mappings.

Link: https://lkml.kernel.org/r/20210323182520.2712101-3-bgeffon@google.com
Signed-off-by: Brian Geffon <bgeffon@google.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: "Michael S . Tsirkin" <mst@redhat.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Sonny Rao <sonnyrao@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Dmitry Safonov <dima@arista.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>