Fix reporting of "Used" space in Btrfs partitions.

There are two numbers found by grepping for "bytes used".
It seems that the first one is more important, e.g. we cannot
resize partitions to a smaller size than this number.

svn path=/trunk/extragear/sysadmin/partitionmanager/; revision=1324119

src/fs/btrfs.cpp

@@ -107,7 +107,7 @@ namespace FS
 
 		if (cmd.run())
 		{
-			QRegExp rxBytesUsed("\nbytes used (\\d+)");
+			QRegExp rxBytesUsed(" bytes used (\\d+)");
 
 			if (rxBytesUsed.indexIn(cmd.output()) != -1)
 				return rxBytesUsed.cap(1).toLongLong();