libceph: initialize mon_client con only once

libceph: initialize mon_client con only once

Do not re-initialize the con on every connection attempt.  When we
ceph_con_close, there may still be work queued on the socket (e.g., to
close it), and re-initializing will clobber the work_struct state.

Signed-off-by: Sage Weil <sage@inktank.com>