xfrm: xfrm_alloc_spi shouldn't use 0 as SPI

In the Linux kernel, the following vulnerability has been resolved:

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.