mm: thp: remove redundant pgtable check in set_huge_zero_page()

When the pgtable is NULL in the set_huge_zero_page(), we should not
increment the count of PTE page table pages by calling mm_inc_nr_ptes(). 
Otherwise we may receive the following warning when the mm exits:

	BUG: non-zero pgtables_bytes on freeing mm

Now we can't observe the above warning since only
do_huge_pmd_anonymous_page() invokes set_huge_zero_page() and the pgtable
can not be NULL.

Therefore, instead of moving mm_inc_nr_ptes() to the non-NULL branch of
pgtable, it is better to remove the redundant pgtable check directly.

Link: https://lkml.kernel.org/r/20220818082748.40021-1-zhengqi.arch@bytedance.comSigned-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/huge_memory.c

@@ -770,7 +770,6 @@ static void set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
 		return;
 	entry = mk_pmd(zero_page, vma->vm_page_prot);
 	entry = pmd_mkhuge(entry);
-	if (pgtable)
 	pgtable_trans_huge_deposit(mm, pmd, pgtable);
 	set_pmd_at(mm, haddr, pmd, entry);
 	mm_inc_nr_ptes(mm);