Return long long from get_zram_mem_used()

Commit 187a6aeb replaced atoll() with
fscanf("%ld").  Though unlikely, there could theoretically be 32-bit
devices with >2GB zram where the type difference matters.

Bug: 25951511

Change-Id: I902e27a214038fea1396185ef9a521316b0009f9
Signed-off-by: Greg Hackmann <ghackmann@google.com>