rbd-fuse: fix enumerate_images() image names buffer size issue

Image names buffer is fixed at 1024.  This turns out to be not enough:
there are at least two "rbd-fuse rbd_list: error %d Numerical result
out of range" reports on the ML.  Fix it by calling rbd_list() twice to
first get the expected buffer size.  Also, get rid of the memory leak
and tweak the error message while at it.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>