libceph: fix mutex coverage for ceph_con_close

libceph: fix mutex coverage for ceph_con_close

(cherry picked from commit 8c50c817566dfa4581f82373aac39f3e608a7dc8)

Hold the mutex while twiddling all of the state bits to avoid possible
races.  While we're here, make not of why we cannot close the socket
directly.

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