kernel-interface: Use reqid as sole key in hash table

kernel-interface: Use reqid as sole key in hash table

Every reqid is allocated once, we don't store the same reqid with e.g.
different marks or interface IDs that would make it necessary to make
them part of the key in that table (that's different in the other table).

To preserve the current behavior, that is, allocating a new reqid if e.g.
the marks are different, the additional selector values (which will
result in an additional policy in the Linux kernel) are compared after
the initial lookup.