MEDIUM: fd: refine the fd_takeover() migration lock

MEDIUM: fd: refine the fd_takeover() migration lock

When haproxy is compiled without double-word CAS, we use a migration lock
in fd_takeover(). This lock was covering the atomic OR on the running_mask
before checking its value, while it is not needed since this atomic op
already returns the result. Let's just refine the code to avoid grabbing
the lock in the event another thread has already stolen the FD, this may
reduce contention in high reuse rate scenarios.