Replace usage of PAGE_SIZE

Per RFC 0016[1], page size in userspace should be a runtime
determination and this constant should go away.

[1] https://fuchsia.dev/fuchsia-src/contribute/governance/rfcs/0016_boot_time_page_sizes

Bug: 42146863
Change-Id: I09aabf29a24d3124b3d914fa86242d3c941297d6
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/android.googlesource.com/platform/system/libfmq/+/1421424
Reviewed-by: James Robinson <jamesr@google.com>