mailbot: Skip upstream commit warnings for stable-only patches

mailbot: Skip upstream commit warnings for stable-only patches

When a commit message contains "stable-only" or "stable only", it indicates
that the patch is specifically created for stable branches and doesn't have
a corresponding upstream commit. In these cases, the mailbot should not warn
about being unable to find the commit upstream.

Suggested-by: Borislav Petkov <bp@alien8.de>
Pinky-sworn-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>