Fix more wrong usermode virtual address types

Fixes warning:
  CC    sparc-bsd-user/exec.o
/src/qemu/exec.c: In function `page_check_range':
/src/qemu/exec.c:2375: warning: comparison is always true due to limited range of data type

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
