Once we remove client_mutex protection, it'll be possible to have an
in-flight operation using an openstateid when a CLOSE call comes in.
If that happens, we can't just put the sc_file reference and clear its
pointer without risking an oops.

Fix this by ensuring that v4.0 CLOSE operations wait for the refcount
to drop before proceeding to do so.
Signed-off-by: NJeff Layton <jlayton@primarydata.com>
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>