bcachefs: Fix race path in bch2_inode_insert()

__destroy_new_inode() is appropriate when we have _just_allocated the
inode, but not when it's been fully initialized and on i_sb_list.

Reported-by:  <syzbot+a0ddc9873c280a4cb18f@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>