net/core: fix kernel-doc warnings

Fix kernel-doc warnings in net/core:

Warning(net/core/skbuff.c:3368): No description found for parameter 'delta_truesize'
Warning(net/core/filter.c:628): No description found for parameter 'pfp'
Warning(net/core/filter.c:628): Excess function parameter 'sk' description in 'sk_unattached_filter_create'

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: David S. Miller <davem@davemloft.net>