Update and new bcachefs fs usage command

Replication tests are finally all passing! This means that device removal and write error handling (for replicated writes) should finally be fully working.

Those two codepaths have in common that they need to modify pointers to existing btree nodes - removing the pointer to the device that either failed to write or is being removed - which is a particularly tricky operation, partially due to the btree node cache being physically indexed. Also fixed a whole bunch of bugs in the code that tracks replicas in the superblock recently, which is relevant to anyone who's using multiple devices.

The only thing left before replication is ready for real use is a tool to rereplicate degraded data (i.e. find all data in a filesystem that has fewer replicas than the desired number of replicas and write more copies). Also want a scrub tool, but that won't be much extra work once there's a userspace interface for these kinds of "do stuff with existing data" jobs.

There's also a new "bcachefs fs usage" command, which reports a whole bunch of information about disk usage. Example output (apologies for formatting):

Filesystem 96e038ff-b680-4d3d-b5fa-8b0ecf4f1ebe:

Size:                       7.4G

Used:                       1.2M

By replicas:                  1x          2x          3x          4x

  btree:                       0      640.0K           0           0

  data:                        0           0           0           0

  cached:                      0           0           0           0

  reserved:                    0           0           0           0

  online reserved:             0

Device 0 usage:                     /dev/sdb   readwrite

                            data     buckets  fragmented

  sb:                     132.0K           2      124.0K

  journal:                 16.0M         128           0

  btree:                  640.0K           5           0

  data:                        0           0           0

  cached:                      0           0           0

Device 1 usage:                     /dev/sdc   readwrite

                            data     buckets  fragmented

  sb:                     132.0K           2      124.0K

  journal:                 16.0M         128           0

  btree:                  640.0K           5           0

  data:                        0           0           0

  cached:                      0           0           0