librbd: drop snap_lock before invalidating cache

Writeback will take the snap_lock, so read everything we need under it
before invalidating the cache. This avoids a recursive lock when writeback
uses snap_lock while snap_rollback() was holding it.

Remove a not-very-useful debugging message that depended on snap_lock being held.

Fixes: #4249
Backport: bobtail
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge remote-tracking branch 'gh/wip-4249' into next