ANDROID: squashfs: Fix signed division issue

The value here can change depending on the type that PAGE_SIZE
has on a given architecture. To avoid the ensuing signed and
unsigned division conversions, we shift instead using PAGE_SHIFT

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Bug: 35257858
Change-Id: I132cae93abea39390c3f0f91a4b2e026e97ed4c7