Btrfs: resume balance on rw (re)mounts properly

This introduces btrfs_resume_balance_async(), which, given that
restriper state was recovered earlier by btrfs_recover_balance(),
resumes balance in btrfs-balance kthread.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 0eb9a4d..e239915 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1187,6 +1187,10 @@
 		if (ret)
 			goto restore;
 
+		ret = btrfs_resume_balance_async(fs_info);
+		if (ret)
+			goto restore;
+
 		sb->s_flags &= ~MS_RDONLY;
 	}