bcachefs: Fix nested transaction restart handling in bch2_bucket_gens_init()

bcachefs: Fix nested transaction restart handling in bch2_bucket_gens_init()

Nested transaction restart handling is typically best avoided; when the
inner context handles a transaction restart it invalidates the outer
transaction context, so we need to make sure to return a
transaction_restart_nested error.

This code wasn't doing that, and hit the assertion in
for_each_btree_key() that checks for that via trans->restart_count.

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