nfsd4: nfsd4_destroy_session must set callback client under the state lock

nfsd4_set_callback_client must be called under the state lock to atomically
set or unset the callback client and shutting down the previous one.

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

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

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