staging: erofs: clean up z_erofs_map_blocks_iter

This patch mainly introduces `vle_map_blocks_iter_ctx' to clean up
z_erofs_map_blocks_iter and vle_get_logical_extent_head.

It changes the return value of `vle_get_logical_extent_head' to int
for the later error handing. In addition, it also renames `pcn' to
`pblk' since only `pblk' exists in erofs compression ondisk format.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>