MINOR: fd: factorize the fd_takeover() exit path to make it safer

MINOR: fd: factorize the fd_takeover() exit path to make it safer

Since there was a risk of leaving fd_takeover() without properly
stopping the fd, let's take this opportunity for factoring the code
around a commont exit point that's common to both double-cas and locked
modes. This means using the "ret" variable inside the double-CAS code,
and inverting the loop to first test the old values. Doing do also
produces cleaner code because the compiler cannot factorize common
exit paths using asm statements that are present in some atomic ops.