NFSD: Fix use-after-free warning when doing inter-server copy

The source file nfsd_file is not constructed the same as other
nfsd_file's via nfsd_file_alloc. nfsd_file_put should not be
called to free the object; nfsd_file_put is not the inverse of
kzalloc, instead kfree is called by nfsd4_do_async_copy when done.

Fixes: ce0887ac

 ("NFSD add nfs4 inter ssc to nfsd4_copy")
Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>