xfrm: remove state and template sort indirections from xfrm_state_afinfo

No module dependency, placing this in xfrm_state.c avoids need for
an indirection.

This also removes the state spinlock -- I don't see why we would need
to hold it during sorting.

This in turn allows to remove the 'net' argument passed to
xfrm_tmpl_sort.  Last, remove the EXPORT_SYMBOL, there are no modular
callers.

For the CONFIG_IPV6=m case, vmlinux size increase is about 300 byte.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>