zram: Fix compilation warning due to print format mismatch

For 64 bit architecture atomic64_t is defined as
long while for 32 bit architecture atomic64_t is
defined as u64. This results in print format
mismatch when we try to print value using
atomic64_read. Fix this compile time warning by
putting required type casting.

Change-Id: I64aaf3936c71755ddbbfacb2bc997f4efba6d3c8
Signed-off-by: Susheel Khiani <skhiani@codeaurora.org>