nfsd: simplify stateid allocation and file handling

nfsd: simplify stateid allocation and file handling

Don't allow stateids to clear the open file pointer until they are
being destroyed. In a later patches we'll want to rely on the fact that
we have a valid file pointer when dealing with the stateid and this
will save us from having to do a lot of NULL pointer checks before
doing so.

Also, move to allocating stateids with kzalloc and get rid of the
explicit zeroing of fields.

Signed-off-by: Trond Myklebust <[email protected]>
Signed-off-by: Jeff Layton <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: J. Bruce Fields <[email protected]>