libceph: protect ceph_con_open() with mutex

libceph: protect ceph_con_open() with mutex

(cherry picked from commit 5469155f2bc83bb2c88b0a0370c3d54d87eed06e)

Take the con mutex while we are initiating a ceph open.  This is necessary
because the may have previously been in use and then closed, which could
result in a racing workqueue running con_work().

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Alex Elder <elder@inktank.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>