mm: fix clear_refs_write locking

[ 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>