kernel/sched: Properly synchronize pend()

Kernel wait_q's and the thread pended_on backpointer are scheduler
state and need to be modified under the scheduler lock.  There was one
spot in pend() where they were not.

Also unpack z_remove_thread_from_ready_q() into an unsynchronized
utility so that it can be called by this process in a single lock
block.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
1 file changed