In the key cache fill path, we use path_upgrade() on a path that isn't
uptodate yet but should be locked.

This change makes bch2_btree_path_upgrade() slightly looser so we can
use it in key cache upgrade, instead of the __ version.

Also, make the related assert - that path->uptodate implies nodes_locked
- slightly clearer.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>