mm/sharepool: Reserve the va space for share_pool

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



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Change the mmap_base in mm_struct and check the limit in
get_unmapped_area.

The vma mapped from sharepool cannot merge because the sp_area cannot
merge. Check this in is_mergeable_vma() instead of vma_merge().

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