erofs: get rid of z_erofs_do_map_blocks() forward declaration

commit 999f2f9a upstream.

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>
Stable-dep-of: 9ed50b82 ("erofs: fix incorrect symlink detection in fast symlink")
Link: https://lore.kernel.org/r/20230204093040.97967-5-hsiangkao@linux.alibaba.com


[ Gao Xiang: apply this to 6.6.y to avoid further backport twists
             due to obsoleted EROFS_BLKSIZ. ]
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>