Fix a gcc warning: to be safe all intermediate pointers in cast from
'...' to '...' must be 'const' qualified [-Wcast-qual].
Doing what is essentially a __DECONST() adding the uintptr_t gets
rid of the massive amount of warnings we get in LinuxKPI and lets
us see the actual problems a lot better.
This is a follow-up to 74e908b3c63b28de1d590dc42502fbe959a6da2e which
Suggested by: jhb
MFC after: 10 days