erofs: simplify erofs_map_blocks_flatmode()

Get rid of redundant variables (nblocks, offset) and a dead branch
(!tailendpacking).

Signed-off-by: Hongzhen Luo <hongzhen@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20240905030339.1474396-1-hongzhen@linux.alibaba.com


Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>