selftests/mm: reduce uffd-unit-test poison test to minimum

The test will still generate quite some unwanted MCE error messages to
syslog.  There was old proposal ratelimiting the MCE messages from kernel,
but that has risk of hiding real useful information on production systems.

We can at least reduce the test to minimum to not over-pollute dmesg,
however trying to not lose its coverage too much.

[peterx@redhat.com: reduce uffd-unit-test poison test to minimum]
  Link: https://lkml.kernel.org/r/aF2RSsjuEOtzXcUa@x1.local
Link: https://lkml.kernel.org/r/20250620150058.1729489-1-peterx@redhat.com


Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>