MINOR: fd: slightly optimize the fd_takeover double-CAS loop

MINOR: fd: slightly optimize the fd_takeover double-CAS loop

The loop in fd_takeover() around the double-CAS is conditionned on
a previous value of old_masks[0] that always matches tid_bit on the
first iteration because it does not result from the atomic op but
from a pre-loaded value. Let's set the result of the atomic op there
instead so that the conflict between threads can be detected earlier
and before performing the double-word CAS.