staging: ashmem: Fix ASHMEM_PURGE_ALL_CACHES return value

Hopefully this isn't too late for 3.12.

In commit 7dc19d5a (convert shrinkers to new count/scan API)
the return value to PURGE_ALL_CACHES was dropped, causing -EPERM
to always be returned.

This patch re-adds the ret assignment, setting it to the the
ashmem_shrink_count(), which is the lru_count.

(Sorry this was missed in the review!)

Fixes: 7dc19d5a ("convert shrinkers to new count/scan API")
Cc: Colin Cross <ccross@android.com>
Cc: Android Kernel Team <kernel-team@android.com>
Cc: Glauber Costa <glommer@openvz.org>
Reported-by: YongQin Liu <yongqin.liu@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Cc: stable <stable@kernel.org> # 3.12
Acked-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>