bcachefs: Assert that we're not violating key cache coherency rules

bcachefs: Assert that we're not violating key cache coherency rules

We're not allowed to have a dirty key in the key cache if the key
doesn't exist at all in the btree - creation has to bypass the key
cache, so that iteration over the btree can check if the key is present
in the key cache.

Things break in subtle ways if cache coherency is broken, so this needs
an assert.

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