net: filter: minor: fix kdoc in __sk_run_filter

This minor patch fixes the following warning when doing
a `make htmldocs`:

  DOCPROC Documentation/DocBook/networking.xml
Warning(.../net/core/filter.c:135): No description found for parameter 'insn'
Warning(.../net/core/filter.c:135): Excess function parameter 'fentry' description in '__sk_run_filter'
  HTML    Documentation/DocBook/networking.html

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>