Crash in multi-thread scenarios

[chenyidu]

Describe the bug

#6265 removed the following code so that ucp_tag_probe_nb() is no longer multithread safe, even if I configured --enable-mt.

In my case, there are multiple threads calling ucp_worker_progress(), and there is another thread executing ucp_tag_probe_nb(). Sometimes, ucp_tag_probe_nb() can not probe the incoming tag message. And sometimes, it just crashes.

In my opinion, this is a bug, and I don't quite understand why these two lines of code were deleted.

[chenyidu]

@brminich Can you tell me why these codes were deleted?

[brminich]

@chenyidu, it is a bug, needs to be reverted back

[brminich]

#7228