mm/gup: Use raw_seqcount_try_begin()

David pointed out that gup_fast() does exactly what the new
raw_seqcount_try_begin() does -- use it.

Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>

mm/gup.c

@@ -3351,8 +3351,7 @@ static unsigned long gup_fast(unsigned long start, unsigned long end,
 		return 0;
 
 	if (gup_flags & FOLL_PIN) {
-		seq = raw_read_seqcount(&current->mm->write_protect_seq);
-		if (seq & 1)
+		if (!raw_seqcount_try_begin(&current->mm->write_protect_seq, seq))
 			return 0;
 	}