bcachefs: fix invalid memory access in bch2_fs_alloc() error path

When bch2_fs_alloc() gets an error before calling
bch2_fs_btree_iter_init(), bch2_fs_btree_iter_exit() makes an invalid
memory access because btree_trans_list is uninitialized.

Signed-off-by: Thomas Bertschinger <tahbertschinger@gmail.com>
Fixes: 6bd68ec2 ("bcachefs: Heap allocate btree_trans")
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>