cachefiles: drop direct usage of ->bmap method.

Replace the direct usage of ->bmap method by a bmap() call.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>