In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

Notice that, in this particular case, the fall-through annotation
is placed at the beginning of the code comment, which is what
GCC is expecting to find.

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enabling
-Wimplicit-fallthrough.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>