fmem: protect qcache statistics with lock

Some of the qcache statistics were being calculated
without being protected by a lock. In very rare cases
this might lead to a nonzero allocation count when
all qcache pages have been freed (and thus must be 0).

Change-Id: I7d82d83d021e36a331fa688e5a7ced7ad8e2b176
CRs-Fixed: 344939
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>