fmq: Remove PAGE_SIZE 4096 usage

bionic hard codes the PAGE_SIZE macro as 4096. This is going away as
Android begins to support larger page sizes. Remove the usage of this
assumption from fmq source; use instead getpagesize() which
provides the real pagesize.

Test: mma
Bug: 295049791
Change-Id: I6ddbd706bc2df561bf7d83dc99ecf84c8d5486e8
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>