bcachefs: Initialize bch_write_op->failed in inline data path

Normally this is initialized in __bch2_write(), which is executed in a
loop, but the inline data path skips this.

Reported-by:  <syzbot+fd3ccb331eb21f05d13b@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>