fs_mgr_overlayfs: Support 16k F2FS

Currently, f2fs block size must be equal to page size.
If this is not the case in the future, this code should be revisited.

Bug: 279820706
Test: adb remount on 16k f2fs device
Change-Id: Ib0d1ea81f921be99471d0433c8499ae077208db7
Signed-off-by: Daniel Rosenberg <drosen@google.com>