block: rbd: fixed may leaks

rbd_client_create() doesn't free rbdc, this leads to many leaks.

seg_len in rbd_do_op() is unsigned, so (seg_len < 0) makes no sense.
Also if fixed check fails then seg_name is leaked.

Signed-off-by: Vasiliy Kulikov <segooon@gmail.com>
Signed-off-by: Yehuda Sadeh <yehuda@hq.newdream.net>