Fix 'get_user_pages_fast()' with non-page-aligned start address

Alexey Dobriyan reported trouble with LTP with the new fast-gup code,
and Johannes Weiner debugged it to non-page-aligned addresses, where the
new get_user_pages_fast() code would do all the wrong things, including
just traversing past the end of the requested area due to 'addr' never
matching 'end' exactly.

This is not a pretty fix, and we may actually want to move the alignment
into generic code, leaving just the core code per-arch, but Alexey
verified that the vmsplice01 LTP test doesn't crash with this.

Reported-and-tested-by: Alexey Dobriyan <[e-mail]>
Debugged-by: Johannes Weiner <[e-mail]>
Cc: Nick Piggin <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/x86/mm/gup.c

···
 			struct page **pages)
 {
 	struct mm_struct *mm = current->mm;
-	unsigned long end = start + (nr_pages << PAGE_SHIFT);
-	unsigned long addr = start;
+	unsigned long addr, len, end;
 	unsigned long next;
 	pgd_t *pgdp;
 	int nr = 0;
 
+	start &= PAGE_MASK;
+	addr = start;
+	len = (unsigned long) nr_pages << PAGE_SHIFT;
+	end = start + len;
 	if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
-					start, nr_pages*PAGE_SIZE)))
+					start, len)))
 		goto slow_irqon;
 
 	/*