nfsd4: use a single struct file for delegations

When we converted to sharing struct filess between nfs4 opens I went too
far and also used the same mechanism for delegations.  But keeping
a reference to the struct file ensures it will outlast the lease, and
allows us to remove the lease with the same file as we added it.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>

diff --cc fs/nfsd/nfs4state.c
Simple merge

diff --cc fs/nfsd/state.h
Simple merge