nfs: is_mounted: match /^nfs.*/ type

This is consistent with the old behavior.

PVE/Storage/NFSPlugin.pm

@@ -22,7 +22,7 @@ sub nfs_is_mounted {
 
     $mountdata = PVE::ProcFSTools::parse_proc_mounts() if !$mountdata;
     return $mountpoint if grep {
-	$_->[2] eq 'nfs' &&
+	$_->[2] =~ /^nfs/ &&
 	$_->[0] =~ m|^\Q$source\E/?$| &&
 	$_->[1] eq $mountpoint
     } @$mountdata;