ANDROID: Incremental fs: Fix INCFS_MAGIC_NUMBER casts

Sparse complains about casting a five byte number to a ulong on 32-bit
platorms. Fix by anding the constant with ULONG_MAX

Bug: 186015158
Test: incfs_test passes, sparse reports no warnings on 32 & 64 bit builds
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: Ic83e03626b7f290370d75b3aaba187b8392fb344