erofs: avoid using generic_block_bmap

Surprisingly, `block' in sector_t indicates the number of
i_blkbits-sized blocks rather than sectors for bmap.

In addition, considering buffer_head limits mapped size to 32-bits,
should avoid using generic_block_bmap.

Link: https://lore.kernel.org/r/20201209115740.18802-1-huangjianan@oppo.com


Fixes: 9da681e0 ("staging: erofs: support bmap")
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Gao Xiang <hsiangkao@redhat.com>
Signed-off-by: Huang Jianan <huangjianan@oppo.com>
Signed-off-by: Guo Weichao <guoweichao@oppo.com>
[ Gao Xiang: slightly update the commit message description. ]
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>