btrfs-progs: Fix over-sized limit on buffer

gcc-4.4 complains (rightly) that the strncpy has a limit too large for
the array it's copying into. Use the correct array length.

Signed-off-by: Hugo Mills <hugo@carfax.org.uk>

btrfs_cmds.c

@@ -427,7 +427,7 @@ int do_clone(int argc, char **argv)
 	}
 
 	args.fd = fd;
-	strncpy(args.name, newname, BTRFS_PATH_NAME_MAX);
+	strncpy(args.name, newname, BTRFS_SUBVOL_NAME_MAX);
 	res = ioctl(fddst, BTRFS_IOC_SNAP_CREATE_V2, &args);
 	e = errno;