ANDROID: Incremental fs: magic number compatible 32-bit

Incfs's magic is bigger than 32-bit, but super block structure's
s_magic is unsigned long which is 32-bit in ARM 32-bit platform.

Do the cast for magic!

Bug: 159772865
Signed-off-by: Peng Zhou <Peng.Zhou@mediatek.com>
Signed-off-by: mtk81325 <peng.zhou@mediatek.com>
Change-Id: Iae4f38774440c7d6ae44529d4f0f8ebb2ec5dacc