btrfs: restrict writes to opened btrfs devices

[FLAG EXCLUSION]
Commit ead62267 ("btrfs: Do not restrict writes to btrfs devices")
removes the BLK_OPEN_RESTRICT_WRITES flag when opening the devices
during mount.  This was an exception at the time as it depended on other
patches.

[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 the 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 had 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 has been 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>
Signed-off-by: David Sterba <dsterba@suse.com>