NFSD fixing possible null pointer derefering in copy offload

commit 18f428d4 upstream.

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>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>