kernel: Fix k_stack_alloc_init behavior

The implementation of this syscall can return either 0 or -ENOMEM, but
when USERSPACE is enabled and it is called through syscall it always
return 0.

Just change this syscall implementation to return the value of
_impl_k_stack_alloc_init

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