ipsec: add ipSecXfrmMark{Set,Get}SPI helpers
ipsec: add ipSecXfrmMark{Set,Get}SPI helpers

which allows to encode and extract an SPI value in and from a XfrmMark

Signed-off-by: Gilberto Bertin <a93f35d43382e9739cc1b4df252abaa0d7c5c77f@cilium.io>

michi-covalent/cilium,cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium