[kernel] Change VMA syscall addr param to inout

This change allows the `vma_map` and `vma_create_map` syscalls to map to
addresses other than the one specified, and therefore makes the address
parameter to those syscalls `inout` in order to return the mapped
address.

Also add the `exact` flag for specifying that mapping needs to be done
at the exact address given. If the mapping collides with another, the
new `j6_err_collision` error is returned.

src/user/drv.uefi_fb/main.cpp

@@ -55,7 +55,7 @@ main(int argc, const char **argv, const char **env)
         return s;
     }
 
-    s = j6_vma_map(fb_handle, 0, lfb_addr);
+    s = j6_vma_map(fb_handle, 0, &lfb_addr, j6_vm_flag_exact);
     if (s != j6_status_ok) {
         return s;
     }