ion: fix the return value of shrinkers

ION page pool and cma shrinkers return the total pooled
memory instead of the freed. This can result in an early
exit of ION page pool shrinker thus giving back less memory
to the system. For cma shrinker this change doesn't have an
effect since the callers of shrink_slab currently ignores
the return value.

Another, it is to fix a corner case when doing drop caches, eg.
"echo 2/3 > /proc/sys/vm/drop_caches". There is an infinite
loop when freed pages > 10.

Change-Id: I0b3727807d9a5f020623766d27da360d077395f3
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
Signed-off-by: Zhenhua Huang <zhenhuah@codeaurora.org>