bcachefs: bcachefs_metadata_version_fast_device_removal

bcachefs: bcachefs_metadata_version_fast_device_removal

Fast device removal, that uses backpointers to find pointers to the
device being removed instead of a full metadata scan.

This requires BCH_SB_MEMBER_DELETED_UUID, which is an incompatible
change - hence the version number bump. We don't fully trust
backpointers, so we don't want to reuse device indexes until after a
fsck has verified that there aren't any pointers to removed devices.

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