net: netfilter: fix compilation warnings

Fix compilation warnings due to type casting between pointers and
function pointers which are treated as errors when the configs
CONFIG_NETFILTER_XT_MATCH_QTAGUID and
CONFIG_NETFILTER_XT_MATCH_QUOTA2 are enabled

CRs-fixed: 638281
Change-Id: Idce65bcaddc68cc58e42951c183a8fa78810f28f
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>