rbd: print info if written length bad

Print info in the event a write comes back short (or any other value
that differs from what was requested).

Signed-off-by: Alex Elder <elder@inktank.com>
loop: print more error info with write errors

Include the result returned by the underlying file's write method if
it differs from the number of bytes requested in the message printed
in __do_lo_send_write().

Similarly, include the value returned by lo_do_transfer() in
do_lo_send_write() in the message printed if it's non-zero.

Signed-off-by: Alex Elder <elder@inktank.com>