nfsd4: make sure sequence flags are set after destroy_session

If this loses any backchannel, make sure we have a chance to notice that
and set the sequence flags.

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

diff --cc fs/nfsd/nfs4callback.c
Simple merge

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

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