md/bitmap: don't set max_write_behind if there is no write mostly device

[ Upstream commit 8c13ab11 ]

We shouldn't set it since write behind IO should only happen to write
mostly device.

Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Signed-off-by: Song Liu <songliubraving@fb.com>
Stable-dep-of: 44abfa6a ("md/md-bitmap: hold 'reconfig_mutex' in backlog_store()")
Signed-off-by: Sasha Levin <sashal@kernel.org>