Remove unnecessary unlock() in Cache.getEntriesFromCache
Remove unnecessary unlock() in Cache.getEntriesFromCache

Motivation:

- Consistent locking/unlocking `c.mu` in `Cache.getEntries`
- Fix CI failure

`Cache.getEntriesFromCache()` was releasing `c.mu` when returning an
entry and not releasing it when returning nil. Previously, the caller
(`Cache.getEntries`) was accordingly not releasing `c.mu` when
`getEntriesFromCache()` was returning an entry, and unlocking `c.mu`
otherwise.

`Cache.getEntries` is the only caller for `getEntriesFromCache`.

After this change c.mu is consistently locked and unlocked in
`getEntries`

kopia/kopia,kopia/kopia,kopia/kopia,kopia/kopia