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This prevents a null pointer dereference when
nfs_idmap_complete_pipe_upcall_locked() calls complete_request_key().

Fixes a regression caused by commit 0cac1202

 (NFSv4: Ensure that
idmap_pipe_downcall sanity-checks the downcall data).
Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>