gtp: set NLM_F_MULTI flag in gtp_genl_dump_pdp()

In drivers/net/gtp.c, gtp_genl_dump_pdp() should set NLM_F_MULTI
flag since it returns multipart message.
This patch adds a new arg "flags" in gtp_genl_fill_info() so that
flags can be set by the callers.

Signed-off-by: Yoshiyuki Kurauchi <ahochauwaaaaa@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>