rbd-fuse: fix for loop in open_rbd_image()

Remove uninitialized usage of 'int i' as i++ from 'for' loop.
The variale 'i' is never used in this loop and initialized
before the next use with 0.

Related warning from clang++:

rbd_fuse/rbd-fuse.c:141:36: warning: variable 'i' is uninitialized
when used here [-Wuninitialized]
        for (im = rbd_images; im != NULL; i++, im = im->next) {

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>