rbd: Fix error recovery in rbd_obj_read_sync()

commit a8d4205623ae965e36c68629db306ca0695a2771 upstream.

When we fail to allocate page vector in rbd_obj_read_sync() we just
basically ignore the problem and continue which will result in an oops
later. Fix the problem by returning proper error.

CC: Yehuda Sadeh <yehuda@inktank.com>
CC: Sage Weil <sage@inktank.com>
CC: ceph-devel@vger.kernel.org
Coverity-id: 1226882
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ilya Dryomov <idryomov@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>