Add a do_div() wrapper macro, lldiv().

Add a do_div() wrapper, lldiv(). The new inline function doesn't modify
the dividend and returns the result of division, so it is useful
in complex expressions, i.e. "return(a/b)" -> "return(lldiv(a,b))"

Signed-off-by: Sergei Poselenov <sposelenov@emcraft.com>