Use ptr_to_kulong instead of explicit casts of pointers to kernel_ureg_t

Use ptr_to_kulong instead of explicit casts of pointers to kernel_ureg_t

Direct casts to kernel_ureg_t will not work after the change
of kernel_ureg_t from unsigned long to kernel_ulong_t.

* defs.h (ptr_to_kulong): New macro.
* block.c (print_blkpg_req): Use it instead of explicit casts
to kernel_ureg_t.
* btrfs.c (btrfs_ioctl): Likewise.
* evdev.c (ff_effect_ioctl): Likewise.
* mmsghdr.c (dumpiov_in_mmsghdr): Likewise.
* msghdr.c (print_struct_msghdr, dumpiov_in_msghdr): Likewise.
* mtd.c (decode_mtd_oob_buf): Likewise.
* printsiginfo.c (printsigval, print_si_info): Likewise.
* scsi.c (print_sg_io_v3_req, print_sg_io_v3_res): Likewise.
* sigaltstack.c (print_stack_t): Likewise.
* sock.c (decode_ifconf): Likewise.
* sysctl.c (SYS_FUNC(sysctl)): Likewise.
* util.c (dumpiov_upto): Likewise.
* v4l2.c (print_v4l2_framebuffer, print_v4l2_ext_control,
print_v4l2_ext_controls): Likewise.