r3336: use a struct ntvfs_async_state to be able to do async chaning of ntvfs modules

r3336: use a struct ntvfs_async_state to be able to do async chaning of ntvfs modules

the idea is that a passthru module can use ntvfs_async_state_push() before
calling ntvfs_next_*() and in the _send function it calls
ntvfs_async_state_pop() and then call the upper layer send_fn itself

- ntvfs_nbench is now fully async

- the ntvfs_map_*() functions and the trans(2) mapping functions are not converted yet

metze