fsetown: Avoid process group lock recursion

Restore the pre-1d874ba4f8ba behaviour of disassociating the current
SIGIO recipient before looking up the specified process or process
group. This avoids a lock recursion in the scenario where a process
group is configured to receive SIGIO for an fd when it has already been
so configured.

Reported by: pho
Tested by: pho
Reviewed by: kib
MFC after: 3 days