Fix warning unitialized variable `ret` in `zbd_report_zones`

[ldonoso]

Initialize variable ret to avoid the following warning:

/libzbd/lib/zbd.c:625:9: error:
variable 'ret' may be uninitialized when used here
[-Werror,-Wconditional-uninitialized]
        return ret;

[damien-lemoal]

Looks good. But please add your Signed-off-by tag to the patch commit message.

[floatious]

Luis, thank you for your contribution!