Fix the following warnings

$ make sgmldocs
...
Warning(include/linux/skbuff.h:283): No description found for parameter 'len,data_len,mac_len,csum'
Warning(include/linux/skbuff.h:283): No description found for parameter 'local_df,cloned,pkt_type,ip_summed'
Warning(include/linux/skbuff.h:283): No description found for parameter 'protocol,security'
...
Warning(include/linux/skbuff.h:283): No description found for
parameter 'head,*data,*tail,*end'
...

by adding support for comma-separated members in structs and unions.
Signed-off-by: Alexey Dobriyan <adobriyan@mail.ru>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>