selftest/vm: add mremap expand merge offset test

selftest/vm: add mremap expand merge offset test

Add a test to assert that we can mremap() and expand a mapping starting
from an offset within an existing mapping.  We unmap the last page in a 3
page mapping to ensure that the remap should always succeed, before
remapping from the 2nd page.

This is additionally a regression test for the issue solved in "mm,
mremap: fix mremap() expanding vma with addr inside vma" and confirmed to
fail prior to the change and pass after it.

Finally, this patch updates the existing mremap expand merge test to check
error conditions and reduce code duplication between the two tests.

[lstoakes@gmail.com: increment num_expand_tests so test doesn't complain about unexpected tests being run]
Link: https://lkml.kernel.org/r/8ff3ba3cadc0b6c1b2688ae5c851bf73aa062d57.1673701836.git.lstoakes@gmail.com
Link: https://lkml.kernel.org/r/02b117a8ffd52acc01dc66c2fb39754f08d92c0e.1672675824.git.lstoakes@gmail.com
Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Jakub Matěna <matenajakub@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>