mm: remove obsolete pgd_offset_gate()

Remove pgd_offset_gate() completely and simply make the single caller use
pgd_offset().

It appears that the gate area resides in the kernel-mapped segment
exclusively on IA64. Therefore, removing pgd_offset_k is safe since IA64
is now obsolete.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Feng Lee <[e-mail]>
Reviewed-by: Barry Song <[e-mail]>
Acked-by: David Hildenbrand <[e-mail]>
Cc: Anshuman Khandual <[e-mail]>
Cc: bibo mao <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jason Gunthorpe <[e-mail]>
Cc: John Hubbard <[e-mail]>
Cc: Lance Yang <[e-mail]>
Cc: Peter Xu <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/pgtable.h

···
 }
 #endif
 
-#ifndef __HAVE_ARCH_PGD_OFFSET_GATE
-#define pgd_offset_gate(mm, addr)	pgd_offset(mm, addr)
-#endif
-
 #ifndef __HAVE_ARCH_MOVE_PTE
 #define move_pte(pte, old_addr, new_addr)	(pte)
 #endif

mm/gup.c

···
 	/* user gate pages are read-only */
 	if (gup_flags & FOLL_WRITE)
 		return -EFAULT;
-	if (address > TASK_SIZE)
-		pgd = pgd_offset_k(address);
-	else
-		pgd = pgd_offset_gate(mm, address);
+	pgd = pgd_offset(mm, address);
 	if (pgd_none(*pgd))
 		return -EFAULT;
 	p4d = p4d_offset(pgd, address);