net/mpls: Implement handler for strict data checking on dumps

Without CONFIG_INET enabled compiles fail with:

net/mpls/af_mpls.o: In function `mpls_dump_routes':
af_mpls.c:(.text+0xed0): undefined reference to `ip_valid_fib_dump_req'

The preference is for MPLS to use the same handler as ipv4 and ipv6
to allow consistency when doing a dump for AF_UNSPEC which walks
all address families invoking the route dump handler. If INET is
disabled then fallback to an MPLS version which can be tighter on
the data checks.

Fixes: e8ba330a ("rtnetlink: Update fib dumps for strict data checking")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>