staging: erofs: support bmap

Add erofs_bmap() to support FIBMAP ioctl on flatmode inode.

Reviewed-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Link: https://lore.kernel.org/r/20190716093256.108791-1-yuchao0@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>