ceph: export ceph_session_state_name function

...so that it can be used from the ceph debugfs
code when dumping session info.

Signed-off-by: John Spray <john.spray@redhat.com>
ceph: additional debugfs output

MDS session state and client global ID is
useful instrumentation when testing.

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #5 from ceph/wip-9466

ceph: additional debugfs output

Reviewed-by: Yan, Zheng <ukernel@gmail.com>