staging: erofs: separate erofs_get_meta_page

This patch separates 'erofs_get_meta_page' into 'erofs_get_meta_page'
and 'erofs_get_meta_page_nofail'. The second one ensures that it
should not fail under memory pressure and should make best efforts
if IO errors occur.

It also adds auxiliary variables in order to fulfill 80 character limit.

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