ipaddress: Abstract IFA_LABEL matching code

ipaddress: Abstract IFA_LABEL matching code

There at least two places in ip/ipaddress.c where we match IFA_LABEL
against filter.label if that is given.

Get rid of "common" if () statement for inet_addr_match_rta() and
ifa_label_match_rta(): it is not common because first will check for
filter.pfx.family != AF_UNSPEC inside and second for filter.label being
non NULL.

This allows us to further simplify down code and prepare for
ll_idx_n2a() replacement with ll_index_to_name() without 80 columns
checkpatch notice.

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>