mm/arm64: Correct obsolete comment in do_page_fault()

commit d8ed45c5dc ("mmap locking API: use coccinelle to convert mmap_sem
rwsem call sites") has convertd down_read_trylock() to mmap_read_trylock().
But it forgot to update the relevant comment.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Link: https://lore.kernel.org/r/20210205090919.63382-1-linmiaohe@huawei.com
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/mm/fault.c

@@ -564,7 +564,7 @@ retry:
 		mmap_read_lock(mm);
 	} else {
 		/*
-		 * The above down_read_trylock() might have succeeded in which
+		 * The above mmap_read_trylock() might have succeeded in which
 		 * case, we'll have missed the might_sleep() from down_read().
 		 */
 		might_sleep();