errata: check kernel-extraversion

When checking a kernel version to see if errata should be set,
also check the kernel extraversion, if given in the errata.txt.
Also, reformat errata.txt to allow for longer version names and
to remove the tabs.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>