netfilter: xtables: change hotdrop pointer to direct modification

Since xt_action_param is writable, let's use it. The pointer to
'bool hotdrop' always worried (8 bytes (64-bit) to write 1 byte!).
Surprisingly results in a reduction in size:

   text    data     bss filename
5457066  692730  357892 vmlinux.o-prev
5456554  692730  357892 vmlinux.o

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>