the functionality of nullfs. The latter is now just a mount & unmount
routine, and a few tables. umapfs borrow most of this infrastructure.
Both fs's are now nfs-exportable.
All layered fs's share a common format to private mount & private
vnode structs (which a particular fs can extend).
Also add genfs_noerr_rele(), a vnode op which will vrele/vput
operand vnodes appropriately.
not in the kernel, genfs_lease_check() is simply a no-op. This allows
LKM'd file systems to be exported (previously did not work properly
due to a compile-time decision based on -DNFSSERVER).
- defopt NFSSERVER