bcachefs: Split out dev_buckets_free()

bcachefs: Split out dev_buckets_free()

Previously, dev_buckets_available() only counted buckets that are
eligible to be allocated right now - i.e. buckets that don't have cached
data, or need discard, or need gc gens, etc.

But most users of this function want to know how many buckets are
eligible to be allocated from without moving data around - copygc,
allocator striping, which means we should be including cached data
buckets etc.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>