staging: erofs: introduce MNGD_MAPPING helper

This patch introduces MNGD_MAPPING to wrap up
sbi->managed_cache->i_mapping, which will be used
to solve too many #ifdefs in a single function.

No logic changes.

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