pstore: don't enable crash_kexec_post_notifiers by default

pstore: don't enable crash_kexec_post_notifiers by default

commit f00c36641a253f4ea659ec3def5d87ba1336eb3b enabled
crash_kexec_post_notifiers by default regardless of whether pstore
is enabled or not.

The original intention to enabled this option by default is that
it only affects kernel post-panic behavior, so should have no harm.
But this is not true if the user wants a reliable kdump.

crash_kexec_post_notifiers is known to cause problem with kdump,
and it's documented in kernel. It's not easy to fix the problem
because of how kdump works. Kdump expects the crashed kernel to
jump to an pre-loaded crash kernel, so doing any extra job before
the jump will increase the risk.

It depends on the user to choose between having a reliable kdump or
some other post-panic debug mechanic.

So it's better to keep this config untouched by default, or it may put
kdump at higher risk of failing silently. User should enable it by
uncommenting the config line manually if pstore is always needed.

Also add a inline comment inform user about the potential issue.

Thanks to Dave Young for finding out this issue.

Fixes #16661

Signed-off-by: Kairui Song <kasong@redhat.com>