bcachefs: Improve bch2_dev_freespace_init()

bcachefs: Improve bch2_dev_freespace_init()

This makes bch2_dev_freespace_init() much faster: instead of processing
every bucket on the device one at a time, we handle ranges of missing
keys all at once: the freespace btree is an extents style btree, so we
only have to insert one freespace key for every range of missing keys
in the alloc btree.

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