NFSD: fix dest to src mount in inter-server COPY

A cleanup of the inter SSC copy needs to call fput() of the source
file handle to make sure that file structure is freed as well as
drop the reference on the superblock to unmount the source server.

Fixes: 36e1e5ba90 ("NFSD: Fix use-after-free warning when doing inter-server copy")
Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: Dai Ngo <dai.ngo@oracle.com>

fs/nfsd/nfs4proc.c

@@ -1302,7 +1302,7 @@ nfsd4_cleanup_inter_ssc(struct vfsmount *ss_mnt, struct nfsd_file *src,
 			struct nfsd_file *dst)
 {
 	nfs42_ssc_close(src->nf_file);
-	/* 'src' is freed by nfsd4_do_async_copy */
+	fput(src->nf_file);
 	nfsd_file_put(dst);
 	mntput(ss_mnt);
 }