md: cleanup mddev flag clear for takeover

Commit 6995f0b (md: takeover should clear unrelated bits) clear
unrelated bits, but it's quite fragile. To avoid error in the future,
define a macro for unsupported mddev flags for each raid type and use it
to clear unsupported mddev flags. This should be less error-prone.

Suggested-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/md.h

@@ -212,6 +212,7 @@ extern int rdev_clear_badblocks(struct md_rdev *rdev, sector_t s, int sectors,
 				int is_new);
 struct md_cluster_info;
 
+/* change UNSUPPORTED_MDDEV_FLAGS for each array type if new flag is added */
 enum mddev_flags {
 	MD_ARRAY_FIRST_USE,	/* First use of array, needs initialization */
 	MD_CLOSING,		/* If set, we are closing the array, do not open
@@ -702,4 +703,11 @@ static inline int mddev_is_clustered(struct mddev *mddev)
 {
 	return mddev->cluster_info && mddev->bitmap_info.nodes > 1;
 }
+
+/* clear unsupported mddev_flags */
+static inline void mddev_clear_unsupported_flags(struct mddev *mddev,
+	unsigned long unsupported_flags)
+{
+	mddev->flags &= ~unsupported_flags;
+}
 #endif /* _MD_MD_H */