kernel: mmu: always use before/after guard pages for k_mem_map()

When we start allowing unmapping of memory region, there is no
exact way to know if k_mem_map() is called with guard page option
specified or not. So just unconditionally enable guard pages on
both sides of the memory region to hopefully catch access
violations.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>