bcachefs: Single device mode

bcachefs: Single device mode

Single device filesystems are now identified by the block device name,
not the UUID - and single device filesystems with the same UUID can be
mounted simultaneously, without any special options.

This allocates a new bit in the superblock, BCH_SB_MULTI_DEVICE, which
indicates whether a filesystem has ever been multi device.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>