btrfs: restrict writes to opened btrfs devices

[WEIRD FLAG EXCLUSION]
Commit ead6226 ("btrfs: Do not restrict writes to btrfs devices")
removes the BLK_OPEN_RESTRICT_WRITES flag when opening the devices
during mount.

However there is no filesystem except btrfs excluding this flag, which
is weird, and lacks a proper explanation why we need such an exception.

[REASON TO EXCLUDE THAT FLAG]
Btrfs needs to call btrfs_scan_one_device() to determine the fsid, no
matter if we're mounting a new fs or an existing one.

But if a fs is already mounted and that BLK_OPEN_RESTRICT_WRITES is
honored, meaning no other write open is allowed for the block device.

Then we want to mount a subvolume of the mounted fs to another mount
point, we will call btrfs_scan_one_device() again, but it will fail due
to the BLK_OPEN_RESTRICT_WRITES flag (no more write open allowed),
causing only one mount point for the fs.

Thus at that time, we have to exclude the BLK_OPEN_RESTRICT_WRITES to
allow multiple mount points for one fs.

[WHY IT'S SAFE NOW]
The root problem is, we do not need to nor should use BLK_OPEN_WRITE for
btrfs_scan_one_device().
That function is only to read out the super block, no write at all, and
BLK_OPEN_WRITE is only going to cause problems for such usage.

The root problem is fixed by patch "btrfs: always open the device
read-only in btrfs_scan_one_device", so btrfs_scan_one_device() will
always work no matter if the device is opened with
BLK_OPEN_RESTRICT_WRITES.

[ENHANCEMENT]
Just remove the btrfs_open_mode(), as the only call site can be replaced
with regular sb_open_mode().

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Qu Wenruo <wqu@suse.com>

Author: adam900710

fs/btrfs/super.c

@@ -261,12 +261,6 @@ static const struct fs_parameter_spec btrfs_fs_parameters[] = {
 	{}
 };
 
-/* No support for restricting writes to btrfs devices yet... */
-static inline blk_mode_t btrfs_open_mode(struct fs_context *fc)
-{
-	return sb_open_mode(fc->sb_flags) & ~BLK_OPEN_RESTRICT_WRITES;
-}
-
 static bool btrfs_match_compress_type(const char *string, const char *type, bool may_have_level)
 {
 	const int len = strlen(type);
@@ -1843,7 +1837,7 @@ static int btrfs_get_tree_super(struct fs_context *fc)
 	struct btrfs_fs_devices *fs_devices = NULL;
 	struct btrfs_device *device;
 	struct super_block *sb;
-	blk_mode_t mode = btrfs_open_mode(fc);
+	blk_mode_t mode = sb_open_mode(fc->sb_flags);
 	int ret;
 
 	btrfs_ctx_to_info(fs_info, ctx);