selftests/mm: specify requirement for PROC_MEM_ALWAYS_FORCE=y

Several of the mm selftests made use of /proc/pid/mem as part of their
operation but we do not specify this in the config fragment for them, at
least mkdirty and ksm_functional_tests have this requirement.

This has been working fine in practice since PROC_MEM_ALWAYS_FORCE was the
default setting but commit 599bbba5a36f ("proc: make PROC_MEM_FORCE_PTRACE
the Kconfig default") that is no longer the case, meaning that tests run
on kernels built based on defconfigs have started having the new more
restrictive default and failing. Add PROC_MEM_ALWAYS_FORCE to the config
fragment for the mm selftests.

Thanks to Aishwarya TCV for spotting the issue and identifying the commit
that introduced it.

Link: https://lore.kernel.org/20260416-selftests-mm-proc-mem-always-force-v1-1-3f5865153c67@kernel.org
Fixes: 599bbba5a36f ("proc: make PROC_MEM_FORCE_PTRACE the Kconfig default")
Signed-off-by: Mark Brown <broonie@kernel.org>
Reported-by: Aishwarya TCV <aishwarya.tcv@arm.com>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>