Revert "net: igmp: increase size of mr_ifc_count"

This reverts commit 4344440d which is
commit b69dd5b3 upstream.

This breaks the kernel abi and is not an issue for Android systems.

Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I1006e8d8ca58cf07ea23404e386ba58651424eb2