mm/sharepool: Implement mg_sp_walk_page_range()

ascend inclusion
category: Feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8LNGH



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This is a simple wrap of walk_page_range() to get all the pages of a
spa. It doesn't support holes.

Signed-off-by: Wang Wensheng <wangwensheng4@huawei.com>