UCP: Add lock to stub_ep pending_add.
UCP: Add lock to stub_ep pending_add.

 This function can be called both from main thread (as part of
non-blocking send operation) or from async context (when sending a
wireup message response), which creates a race condition on the pending
queue.

abouteiller/ucx,shssf/ucx,sergsagal1/ucx_rocm,sergsagal1/ucx_rocm,abouteiller/ucx,shssf/ucx,abouteiller/ucx,abouteiller/ucx,sergsagal1/ucx_rocm,shssf/ucx