nfsd: Don't garbage collect files that might contain write errors

If a file may contain unstable writes that can error out, then we want
to avoid garbage collecting the struct nfsd_file that may be
tracking those errors.
So in the garbage collector, we try to avoid collecting files that aren't
clean. Furthermore, we avoid immediately kicking off the garbage collector
in the case where the reference drops to zero for the case where there
is a write error that is being tracked.

If the file is unhashed while an error is pending, then declare a
reboot, to ensure the client resends any unstable writes.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>