Fix no space left on device issue in tmpfs.
2021-12-01T06:30:40.1651742Z cp: write error: No space left on device
2021-12-01T06:30:40.1652225Z Failure: local_fs_run():/dev/vdb Unable to copy /tmp/tmp.gl4Sgp/onie-installer.bin to tmpfs
Build failed on a Ubuntu 20.04 system with kvm kernel, which does not have the /proc/sys/vm/compact_memory
Should check if compact_memory is writeable before doing it.
Signed-off-by: Chris Ward <tjcw@uk.ibm.com>
