mm/sharepool: Reorganize create_spg()

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I650K6



--------------------------------

1. Extract a function that initialize all the members for a newly
   allocated sp_group. Just to decrease the function size.
2. Move the idr_alloc to the end of the function, since we should not
   add an uninitialized sp_group to the global idr.
3. Rename the file for hugetlb map.

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