utils: Introduce get_addr_rta() and inet_addr_match_rta()

utils: Introduce get_addr_rta() and inet_addr_match_rta()

First is used to get address from netlink attribute to
inet_prefix data structure. Use memcpy() with constant
value to let complier optimize by replacing a call by
inlining load/store instructions.

Second is used to match address in given netlink attribute
with one given as reference. It matches successfully if
no attribute is given (@rta is NULL), reference address
family is AF_UNSPEC or it's length isn't given; fails if
get_attr_rta() can't get attribute or it's family does
not match reference; calls inet_addr_match() to get final
verdict.

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