Add an explicit type parameter to the P2ROUNDUP() macro

Add an explicit type parameter to the P2ROUNDUP() macro

Because it uses signed logic internally it is currently quite vulnerable to
mismatched argument types leading to weird evaluations. Therefore it's safer
to give it an explicit type parameter and have it cast its other arguments to
that type.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>