nfsd: implement OPEN_ARGS_SHARE_ACCESS_WANT_OPEN_XOR_DELEGATION

Allow clients to request getting a delegation xor an open stateid if a
delegation isn't available. This allows the client to avoid sending a
final CLOSE for the (useless) open stateid, when it is granted a
delegation.

If this flag is requested by the client and there isn't already a new
open stateid, discard the new open stateid before replying.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>