staging: erofs: cleanup `z_erofs_vle_normalaccess_readpages'

This patch introduces `__should_decompress_synchronously' to
cleanup `z_erofs_vle_normalaccess_readpages'.

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