erofs: get rid of z_erofs_do_map_blocks() forward declaration

The code can be neater without forward declarations.  Let's
get rid of z_erofs_do_map_blocks() forward declaration.

Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230204093040.97967-5-hsiangkao@linux.alibaba.com