NFSD fixing possible null pointer derefering in copy offload

Static checker revealed possible error path leading to possible
NULL pointer dereferencing.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: e0639dc5: ("NFSD introduce async copy feature")
Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>