The "ret = bkey_err(k);" assignment was accidentally left out so the
call to bch2_btree_iter_peek_slot() is not checked for errors.

Fixes: 53306e096d91 ("bcachefs: Always check for transaction restarts")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>