erofs: remove linux/buffer_head.h dependency

EROFS actually never uses buffer heads, therefore just get rid of
BH_xxx definitions and linux/buffer_head.h inclusive.

Link: https://lore.kernel.org/r/20230113065226.68801-2-hsiangkao@linux.alibaba.com


Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>