md: introduce md_ro_state

[ Upstream commit f97a5528

 ]

Introduce md_ro_state for mddev->ro, so it is easy to understand.

Signed-off-by: Ye Bin <yebin10@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Stable-dep-of: c9f7cb5b

 ("md: don't leave 'MD_RECOVERY_FROZEN' in error path of md_set_readonly()")
Signed-off-by: Sasha Levin <sashal@kernel.org>