lib/assoc_array.c: mark expected switch fall-through

lib/assoc_array.c: mark expected switch fall-through

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

This patch fixes the following warning:

  lib/assoc_array.c: In function `assoc_array_delete':
  lib/assoc_array.c:1110:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
     for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
     ^~~
  lib/assoc_array.c:1118:2: note: here
    case assoc_array_walk_tree_empty:
    ^~~~

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

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Link: http://lkml.kernel.org/r/20190212212206.GA16378@embeddedor
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>