net/core: Fix rtnetlink kernel-doc headers

This patch avoids that the following warnings are reported when building
with W=1:

net/core/rtnetlink.c:3580: warning: Function parameter or member 'ndm' not described in 'ndo_dflt_fdb_add'
net/core/rtnetlink.c:3580: warning: Function parameter or member 'tb' not described in 'ndo_dflt_fdb_add'
net/core/rtnetlink.c:3580: warning: Function parameter or member 'dev' not described in 'ndo_dflt_fdb_add'
net/core/rtnetlink.c:3580: warning: Function parameter or member 'addr' not described in 'ndo_dflt_fdb_add'
net/core/rtnetlink.c:3580: warning: Function parameter or member 'vid' not described in 'ndo_dflt_fdb_add'
net/core/rtnetlink.c:3580: warning: Function parameter or member 'flags' not described in 'ndo_dflt_fdb_add'
net/core/rtnetlink.c:3718: warning: Function parameter or member 'ndm' not described in 'ndo_dflt_fdb_del'
net/core/rtnetlink.c:3718: warning: Function parameter or member 'tb' not described in 'ndo_dflt_fdb_del'
net/core/rtnetlink.c:3718: warning: Function parameter or member 'dev' not described in 'ndo_dflt_fdb_del'
net/core/rtnetlink.c:3718: warning: Function parameter or member 'addr' not described in 'ndo_dflt_fdb_del'
net/core/rtnetlink.c:3718: warning: Function parameter or member 'vid' not described in 'ndo_dflt_fdb_del'
net/core/rtnetlink.c:3861: warning: Function parameter or member 'skb' not described in 'ndo_dflt_fdb_dump'
net/core/rtnetlink.c:3861: warning: Function parameter or member 'cb' not described in 'ndo_dflt_fdb_dump'
net/core/rtnetlink.c:3861: warning: Function parameter or member 'filter_dev' not described in 'ndo_dflt_fdb_dump'
net/core/rtnetlink.c:3861: warning: Function parameter or member 'idx' not described in 'ndo_dflt_fdb_dump'
net/core/rtnetlink.c:3861: warning: Excess function parameter 'nlh' description in 'ndo_dflt_fdb_dump'

Cc: Hubert Sokolowski <hubert.sokolowski@intel.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: David S. Miller <davem@davemloft.net>