bch2_btree_key_cache_drop() evicts the key cache entry - it's used when
we're doing an update that bypasses the key cache, because for cache
coherency reasons a key can't be in the key cache unless it also exists
in the btree - i.e. creates have to bypass the cache.

After evicting, the path no longer points to a key cache key, and
relock() will always fail if should_be_locked is true.

Prep for improving path->should_be_locked assertions
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>