mm/pgtable: define pte_index so that preprocessor could recognize it

Since commit 974b9b2c ("mm: consolidate pte_index() and
pte_offset_*() definitions") pte_index is a static inline and there is
no define for it that can be recognized by the preprocessor.  As a
result, vm_insert_pages() uses slower loop over vm_insert_page() instead
of insert_pages() that amortizes the cost of spinlock operations when
inserting multiple pages.

Link: https://lkml.kernel.org/r/20220111145457.20748-1-rppt@kernel.org
Fixes: 974b9b2c

 ("mm: consolidate pte_index() and pte_offset_*() definitions")
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Reported-by: Christian Dietrich <stettberger@dokucode.de>
Reviewed-by: Khalid Aziz <khalid.aziz@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>