xfrm: xfrm_alloc_spi shouldn't use 0 as SPI

xfrm: xfrm_alloc_spi shouldn't use 0 as SPI

x->id.spi == 0 means "no SPI assigned", but since commit
94f39804d891 ("xfrm: Duplicate SPI Handling"), we now create states
and add them to the byspi list with this value.

__xfrm_state_delete doesn't remove those states from the byspi list,
since they shouldn't be there, and this shows up as a UAF the next
time we go through the byspi list.

Reported-by: syzbot+a25ee9d20d31e483ba7b@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=a25ee9d20d31e483ba7b
Fixes: 94f39804d891 ("xfrm: Duplicate SPI Handling")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>