libceph: SOCK_CLOSED is a flag, not a state

The following commit changed it so SOCK_CLOSED bit was stored in
a connection's new "flags" field rather than its "state" field.

    libceph: start separating connection flags from state
    commit 928443cd

That bit is used in con_close_socket() to protect against setting an
error message more than once in the socket event handler function.

Unfortunately, the field being operated on in that function was not
updated to be "flags" as it should have been.  This fixes that
error.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>