bcachefs: Rewrite bch2_bucket_alloc_new_fs()

bcachefs: Rewrite bch2_bucket_alloc_new_fs()

This changes bch2_bucket_alloc_new_fs() to a simple bump allocator that
doesn't need to use the in memory bucket array, part of a larger patch
series to entirely get rid of the in memory bucket array, except for
gc/fsck.

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