mm: fix clear_refs_write locking

stable inclusion
from stable-5.10.9
commit 41b0b0c09e974ff9760db396097169a1891460d2
bugzilla: 47457

--------------------------------

[ Upstream commit 29a951df ]

Turning page table entries read-only requires the mmap_sem held for
writing.

So stop doing the odd games with turning things from read locks to write
locks and back.  Just get the write lock.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>