ceph: do not touch_caps while iterating over caps list

Avoid confusing iterate_session_caps(), flag the session while we are
iterating so that __touch_cap does not rearrange items on the list.

All other modifiers of session->s_caps do so under the protection of
s_mutex.

Signed-off-by: Sage Weil <sage@newdream.net>

diff --cc fs/ceph/caps.c
Simple merge

diff --cc fs/ceph/mds_client.c
Simple merge

diff --cc fs/ceph/mds_client.h
Simple merge