nfsd4: keep finer-grained callback status

Distinguish between when the callback channel is known to be down, and
when it is not yet confirmed.  This will be useful in the 4.1 case.

Also, we don't seem to be using the fact that this field is atomic.

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

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