selftests: vm: add process_mrelease tests

Introduce process_mrelease syscall sanity tests which include tests
which expect to fail:

- process_mrelease with invalid pidfd and flags inputs
- process_mrelease on a live process with no pending signals

and valid process_mrelease usage which is expected to succeed.  Because
process_mrelease has to be used against a process with a pending SIGKILL,
it's possible that the process exits before process_mrelease gets called. 
In such cases we retry the test with a victim that allocates twice more
memory up to 1GB.  This would require the victim process to spend more
time during exit and process_mrelease has a better chance of catching the
process before it exits and succeeding.

On success the test reports the amount of memory the child had to allocate
for reaping to succeed.  Sample output:

$ mrelease_test
Success reaping a child with 1MB of memory allocations

On failure the test reports the failure. Sample outputs:

$ mrelease_test
All process_mrelease attempts failed!

$ mrelease_test
process_mrelease: Invalid argument

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Suren Baghdasaryan <[email protected]>
Reviewed-by: Shuah Khan <[email protected]>
Acked-by: Christian Brauner (Microsoft) <[email protected]>
Reviewed-by: Muhammad Usama Anjum <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: John Hubbard <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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