staging: erofs: add the missing __init tags

Append __init to `erofs_init_inode_cache',
`z_erofs_init_zip_subsystem' and move these
declarations to internal.h.

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