EROFS actually never uses buffer heads, therefore just get rid of
BH_xxx definitions and linux/buffer_head.h inclusive.
Reviewed-by: Yue Hu <huyue2@xxxxxxxxxxx>
Reviewed-by: Jingbo Xu <jefflexu@xxxxxxxxxxxxxxxxx>
Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>