staging: zcache: fix serialization bug in zv stats

In a multithreaded workload, the zv_curr_dist_counts
and zv_cumul_dist_counts statistics are being corrupted
because the increments and decrements in zv_create
and zv_free are not atomic.

This patch converts these statistics and their corresponding
increments/decrements/reads to atomic operations.

Based on v3.2-rc7

Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>