s390/mm: use pmd_pgtable_page() helper in __gmap_segment_gaddr()

In __gmap_segment_gaddr() pmd level page table page is being extracted
from the pmd pointer, similar to pmd_pgtable_page() implementation.  This
reduces some redundancy by directly using pmd_pgtable_page() instead,
though first making it available.

Link: https://lkml.kernel.org/r/20221125034502.1559986-1-anshuman.khandual@arm.comSigned-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/s390/mm/gmap.c

@@ -336,12 +336,11 @@ static int gmap_alloc_table(struct gmap *gmap, unsigned long *table,
 static unsigned long __gmap_segment_gaddr(unsigned long *entry)
 {
 	struct page *page;
-	unsigned long offset, mask;
+	unsigned long offset;
 
 	offset = (unsigned long) entry / sizeof(unsigned long);
 	offset = (offset & (PTRS_PER_PMD - 1)) * PMD_SIZE;
-	mask = ~(PTRS_PER_PMD * sizeof(pmd_t) - 1);
-	page = virt_to_page((void *)((unsigned long) entry & mask));
+	page = pmd_pgtable_page((pmd_t *) entry);
 	return page->index + offset;
 }
 

include/linux/mm.h

@@ -2510,7 +2510,7 @@ static inline void pgtable_pte_page_dtor(struct page *page)
 
 #if USE_SPLIT_PMD_PTLOCKS
 
-static struct page *pmd_pgtable_page(pmd_t *pmd)
+static inline struct page *pmd_pgtable_page(pmd_t *pmd)
 {
 	unsigned long mask = ~(PTRS_PER_PMD * sizeof(pmd_t) - 1);
 	return virt_to_page((void *)((unsigned long) pmd & mask));