fibmap: Use bmap instead of ->bmap method in ioctl_fibmap

Now we have the possibility of proper error return in bmap, use bmap()
function in ioctl_fibmap() instead of calling ->bmap method directly.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>