net/ipv4: fib_trie: Avoid cryptic ternary expressions

empty_child_inc/dec() use the ternary operator for conditional
operations. The conditions involve the post/pre in/decrement
operator and the operation is only performed when the condition
is *not* true. This is hard to parse for humans, use a regular
'if' construct instead and perform the in/decrement separately.

This also fixes two warnings that are emitted about the value
of the ternary expression being unused, when building the kernel
with clang + "kbuild: Remove unnecessary -Wno-unused-value"
(https://lore.kernel.org/patchwork/patch/1089869/

):

CC      net/ipv4/fib_trie.o
net/ipv4/fib_trie.c:351:2: error: expression result unused [-Werror,-Wunused-value]
        ++tn_info(n)->empty_children ? : ++tn_info(n)->full_children;

Fixes: 95f60ea3 ("fib_trie: Add collapse() and should_collapse() to resize")
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>