ARM: fix warnings about atomic64_read

Fix:
net/netfilter/xt_connbytes.c: In function 'connbytes_mt':
net/netfilter/xt_connbytes.c:43: warning: passing argument 1 of 'atomic64_read' discards qualifiers from pointer target type
...

by adding the missing const.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>