bcachefs: BCH_FEATURE_no_alloc_info

bcachefs: BCH_FEATURE_no_alloc_info

If a filesystem is going to only be used read-only, and will be a
deployable image, we can strip out alloc info for a substantial
reduction in metadata size - around half, due to backpointers.

Alloc info will be regenerated on first read-write mount.

Remounting RW is disallowed for now, since we don't yet have
check_allocations running in RW mode.

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