kbfs_ops: avoid a deadlock on concurrent TeamNameChanged calls
kbfs_ops: avoid a deadlock on concurrent TeamNameChanged calls

Deadlock:
* First call to `KBFSOps.TeamNameChanged` takes `opsLock`.
* Calls `go FolderBranchOps.TeamNameChanged()`.
* Releases `opsLock`.
* `FolderBranchOps.TeamNameChanged()` takes `headLock`.
* Second call to `KBFSOps.TeamNameChanged` takes `opsLock`.
* Calls `FolderBranchOps.getRootNode`, which blocks trying to acquire `headLock`.
* `FolderBranchOps.TeamNameChanged()` calls `KBFSOps.changeHandle()`.
* `KBFSOps.changeHandle()` tries to acquire `opsLock`.
* DEADLOCK.

Instead, make sure we don't hold `opsLock` when calling `getRootNode`.

Issue: KBFS-2565

keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs