bcachefs: Fix inode early destruction path

discard_new_inode() is the wrong interface to use when we need to free
an inode that was never inserted into the inode hash table; we can
bypass the whole iput() -> evict() path and replace it with
__destroy_inode(); kmem_cache_free() - this fixes a WARN_ON() about
I_NEW.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>