lxc: Only add /dev/ashmem to config_nodes when it exists

[JamiKettunen]

This hasn't existed in mainline Linux kernels since v5.18 and trying to always mount it regardless just adds to the noise in waydroid log in most cases, so simply don't add it to config_nodes unless it actually exists.

Silences the following:

lxc-start: waydroid: ../src/lxc/utils.c: safe_mount: 1221 No such file or directory - Failed to mount "/dev/ashmem" onto "/usr/lib/lxc/rootfs/dev/ashmem"

[cniw]

Fixes: #661

[JamiKettunen]

Hm I now wonder if helpers.drivers.probeAshmemDriver(args) should be called before make_entry("/dev/ashmem") so it could be potentially picked up during the config creation in case the module wasn't already probed, or if there is any other way this could be silenced on systems where ashmem is irrelevant

[aleasto]

Hm I now wonder if helpers.drivers.probeAshmemDriver(args) should be called before make_entry("/dev/ashmem") so it could be potentially picked up during the config creation in case the module wasn't already probed

Yeah do that.
Discussion in AOSP seems to be moving towards enabling sys.use_memfd by default but also keep ashmem to support applications that use /dev/ashmem directly

[aleasto]

works