This converts bch2_gc_stripes_done() and bch2_gc_reflink_done() to the
new for_each_btree_key_commit() macro.

The new for_each_btree_key2() and for_each_btree_key_commit() macros
handles transaction retries, allowing us to avoid nested transactions -
which we want to avoid since they're tricky to do completely correctly
and upcoming assertions are going to be checking for that.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>