gfs2_grow: Don't use PATH_MAX in main_grow
    
    Although "large enough" for most cases, PATH_MAX is an arbitrary limit
    for a file path and doesn't reflect reality. Use asprintf(3) to allocate
    only as much memory as we need to hold the path to the rindex. open(2)
    will tell us if it's too long.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>