net-next: Fix warning while make xmldocs caused by skbuff.c

This patch fix following warnings.

.//net/core/skbuff.c:407: warning: No description found
for parameter 'len'
.//net/core/skbuff.c:407: warning: Excess function parameter
 'length' description in '__netdev_alloc_skb'
.//net/core/skbuff.c:476: warning: No description found
 for parameter 'len'
.//net/core/skbuff.c:476: warning: Excess function parameter
'length' description in '__napi_alloc_skb'

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>