NFSD: Fix potential use-after-free in nfsd_file_put()

nfsd_file_put_noref() can free @nf, so don't dereference @nf
immediately upon return from nfsd_file_put_noref().
Suggested-by: Trond Myklebust <trondmy@hammerspace.com>
Fixes: 99939792 ("nfsd: Clean up nfsd_file_put()")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/filecache.c

@@ -308,11 +308,12 @@ nfsd_file_put(struct nfsd_file *nf)
 	if (test_bit(NFSD_FILE_HASHED, &nf->nf_flags) == 0) {
 		nfsd_file_flush(nf);
 		nfsd_file_put_noref(nf);
-	} else {
+	} else if (nf->nf_file) {
 		nfsd_file_put_noref(nf);
-		if (nf->nf_file)
 		nfsd_file_schedule_laundrette();
-	}
+	} else
+		nfsd_file_put_noref(nf);
+
 	if (atomic_long_read(&nfsd_filecache_count) >= NFSD_FILE_LRU_LIMIT)
 		nfsd_file_gc();
 }