fifo: Explicitly initialize generation numbers when opening

The fi_rgen and fi_wgen fields are generation numbers used when sleeping
waiting for the other end of the fifo to be opened. The fields were not
explicitly initialized after allocation, but this was harmless. To
avoid false positives from KMSAN, though, ensure that they get
initialized to zero.

Reported by: KMSAN
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation