net: Rename net_is_xxx...() functions to net_xxx_is...()

Unify the function naming for various network checking functions.

For example:
     net_is_ipv6_addr_loopback() -> net_ipv6_is_addr_loopback()
     net_is_my_ipv6_maddr() -> net_ipv6_is_my_maddr()
etc.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>