units: Don't mount tracefs in LXC

Mounting tracefs fails in unprivileged LXC containers and latest LXD
(3.23) bind mounts tracefs already.

Fixes https://github.com/lxc/lxd/issues/7059

src/core/swap.c

@@ -207,7 +207,7 @@ static int swap_add_device_dependencies(Swap *s) {
                 return 0;
 
         p = swap_get_parameters(s);
-        if (!p)
+        if (!p || !p->what)
                 return 0;
 
         mask = s->from_proc_swaps ? UNIT_DEPENDENCY_PROC_SWAP : UNIT_DEPENDENCY_FILE;

units/sys-kernel-tracing.mount

@@ -12,6 +12,7 @@ Description=Kernel Trace File System
 Documentation=https://www.kernel.org/doc/Documentation/trace/ftrace.txt
 Documentation=https://www.freedesktop.org/wiki/Software/systemd/APIFileSystems
 DefaultDependencies=no
+ConditionVirtualization=!lxc
 ConditionPathExists=/sys/kernel/tracing
 ConditionCapability=CAP_SYS_RAWIO
 Before=sysinit.target