mm: make mmap_region() internal

mm: make mmap_region() internal

Now that we have removed the one user of mmap_region() outside of mm, make
it internal and add it to vma.c so it can be userland tested.

This ensures that all external memory mappings are performed using the
appropriate interfaces and allows us to modify memory mapping logic as we
see fit.

Additionally expand test stubs to allow for the mmap_region() code to
compile and be userland testable.

Link: https://lkml.kernel.org/r/de5a3c574d35c26237edf20a1d8652d7305709c9.1735819274.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Cc: Jann Horn <jannh@google.com>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>