netfilter: fix seq_printf type mismatch warning

The return type of atomic64_read() varies depending on arch.  The
arm64 version is being changed from long long to long in the mainline
for v3.16, causing a seq_printf type mismatch (%llu) in
guid_ctrl_proc_show().

This commit fixes the type mismatch by casting atomic64_read() to u64.

Change-Id: Iae0a6bd4314f5686a9f4fecbe6203e94ec0870de
Signed-off-by: Sherman Yin <shermanyin@gmail.com>