lib: posix: tag the fallthrough case as __fallthrough

Use the __fallthrough directive on the switch fallthrough case and drop
the comment instead. This informs the compiler that the fallthrough is
intentional and silence any possible warning about it.

Drop the not reached case as I think that that part can actually be
reached since there are breaks in some of the cases.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>