* sftp: refuse mkdir and uploads under a missing folder
* sftp: re-check the parent directory when a buffered upload commits
The open-time check cannot catch a parent removed while the upload
buffered; the filer would recreate it when the entry lands.
* sftp: cover mkdir and upload commit against a missing parent
Extend the test filer with CreateEntry so makeDir can be exercised end
to end.
* sftp: refuse a rename into a missing folder
The filer's rename path creates the target entry through CreateEntry,
which recreates absent parents the same way uploads did.
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Co-authored-by: user.email <chrislusf@users.noreply.github.com>
