stable: Allow merging of backports for serious user-visible performance issues

Distribution kernel maintainers routinely backport fixes for users that
were deemed important but not "something critical" as defined by the
rules. To users of these kernels they are very serious and failing to fix
them reduces the value of -stable.

The problem is that the patches fixing these issues are often subtle and
prone to regressions in other ways and need greater care and attention.
To combat this, these "serious" backports should have a higher barrier
to entry.

This patch relaxes the rules to allow a distribution maintainer to merge
to -stable a backported patch or small series that fixes a "serious"
user-visible performance issue. They should include additional information on
the user-visible bug affected and a link to the bugzilla entry if available.
The same rules about the patch being already in mainline still apply.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>