ipv4: Extend FIB alias find function

Extend the function with another argument, 'find_first'. When set, the
function returns the first FIB alias with the matching {prefix, prefix
length, table ID}. The TOS and priority parameters are ignored. Current
callers are converted to pass 'false' in order to maintain existing
behavior.

This will be used by subsequent patches in the series.

v2:
* New patch

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Suggested-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>