mm/hugetlb: remove unnecessary 'NULL' values from pointer

Pointer variables allocate memory first, and then judge.  There is no need
to initialize the assignment.

Link: https://lkml.kernel.org/r/20220914012113.6271-1-xupengfei@nfschina.com


Signed-off-by: XU pengfei <xupengfei@nfschina.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>