NFSv4: Don't free slots prematurely if requesting a directory delegation

NFSv4: Don't free slots prematurely if requesting a directory delegation

When requesting a directory delegation, it is imperative to hold the
slot until the delegation state has been recorded. Otherwise, if a
recall comes in, the call to referring_call_exists() will assume the
processing is done, and when it doesn't find a delegation, it will
assume it has been returned.

Fixes: 156b09482933 ("NFS: Request a directory delegation on ACCESS, CREATE, and UNLINK")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>