mm/arm64: Correct obsolete comment in do_page_fault()

commit d8ed45c5 ("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>