igmp: uninline ip_mc_validate_checksum()

This function is only used via function pointer.

"inline" doesn't hurt given that taking address of an inline function
forces out-of-line version but it doesn't help either.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>