netfilter: xtables: merge xt_MARK into xt_mark

Two arguments for combining the two:
- xt_mark is pretty useless without xt_MARK
- the actual code is so small anyway that the kmod metadata and the module
  in its loaded state totally outweighs the combined actual code size.

i586-before:
-rw-r--r-- 1 jengelh users 3821 Feb 10 01:01 xt_MARK.ko
-rw-r--r-- 1 jengelh users 2592 Feb 10 00:04 xt_MARK.o
-rw-r--r-- 1 jengelh users 3274 Feb 10 01:01 xt_mark.ko
-rw-r--r-- 1 jengelh users 2108 Feb 10 00:05 xt_mark.o
   text    data     bss     dec     hex filename
    354     264       0     618     26a xt_MARK.o
    223     176       0     399     18f xt_mark.o
And the runtime size is like 14 KB.

i586-after:
-rw-r--r-- 1 jengelh users 3264 Feb 18 17:28 xt_mark.o

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