bcachefs: Split out struct gc_stripe from struct stripe

bcachefs: Split out struct gc_stripe from struct stripe

We have two radix trees of stripes - one that mirrors some information
from the stripes btree in normal operation, and another that GC uses to
recalculate block usage counts.

The normal one is now only used for finding partially empty stripes in
order to reuse them - the normal stripes radix tree and the GC stripes
radix tree are used significantly differently, so this patch splits them
into separate types.

In an upcoming patch we'll be replacing c->stripes with a btree that
indexes stripes by the order we want to reuse them.

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