libceph: verify state after retaking con lock after dispatch

libceph: verify state after retaking con lock after dispatch

(cherry picked from commit 7b862e07b1a4d5c963d19027f10ea78085f27f9b)

We drop the con mutex when delivering a message.  When we retake the
lock, we need to verify we are still in the OPEN state before
preparing to read the next tag, or else we risk stepping on a
connection that has been closed.

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