devtools: relax tag checking in fixes

devtools: relax tag checking in fixes

The tag "Cc: stable@dpdk.org" must be set when the commit must be
backported to a stable branch. The reminder is reworded.

It should be located just below the "Fixes:" tag (without blank line)
and followed by a blank line, separated from SoB and review tags below.
However, there is no strong need for checking blank lines.

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Reviewed-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>