bcachefs: Fix I_NEW warning in race path in bch2_inode_insert()

discard_new_inode() is the correct interface for tearing down an indoe
that was fully created but not made visible to other threads, but it
expects I_NEW to be set, which we don't use.

Reported-by: https://github.com/koverstreet/bcachefs/issues/690
Fixes: bcachefs: Fix race path in bch2_inode_insert()
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>