rbd: clean up a few things in the refresh path

This includes a few relatively small fixes I found while examining
the code that refreshes image information.

This resolves:
    http://tracker.ceph.com/issues/5040

Signed-off-by: Alex Elder <elder@inktank.com>
rbd: protect against duplicate client creation

If more than one rbd image has the same ceph cluster configuration
(same options, same set of monitors, same keys) they normally share 
a single rbd client.

When an image is getting mapped, rbd looks to see if an existing
client can be used, and creates a new one if not.

The lookup and creation are not done under a common lock though, so
mapping two images concurrently could lead to duplicate clients
getting set up needlessly.  This isn't a major problem, but it's
wasteful and different from what's intended.

This patch fixes that by using the control mutex to protect
both the lookup and (if needed) creation of the client.  It
was previously used just when creating.

This resolves:
    http://tracker.ceph.com/issues/3094

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: print more info for short message header

If an osd client response message arrives that has a front section
that's too big for the buffer set aside to receive it, a warning
gets reported and a new buffer is allocated.

The warning says nothing about which connection had the problem.
Add the peer type and number to what gets reported, to be a bit more
informative.

Signed-off-by: Alex Elder <elder@inktank.com>