null: drop unneeded casts from void* to non-void*

null: drop unneeded casts from void* to non-void*

Some functions in null ioengine are used by C/C++ (e.g. null_init()
which calls malloc(3)), but C specific ones (not __cplusplus) don't
need explicit cast from void* to non-void*, and one usually doesn't.

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>