kernel: pipe: user threads may not re-init pipe

Updates z_vrfy_k_pipe_init() to use K_SYSCALL_OBJ_NEVER_INIT()
instead of K_SYSCALL_OBJ() to prevent a user thread from
re-initializing a pipe. This aligns the pipe initialization
behavior to that of other kernel objects such as message queues.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>