syscalls: arm: Fix possible overflow in is_in_region function

This function is widely used by functions that validate memory
buffers. Macros used to check permissions, like Z_SYSCALL_MEMORY_READ
and Z_SYSCALL_MEMORY_WRITE, use these functions to check that a
pointers passed by user threads in a syscall.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>