metadata: follow 3.17 changes for current->start_time

As of Linux kernel commit 57e0be041d9e21a73 ("sched: Make
task->real_start_time nanoseconds based"), current->start_time is
stored in nanoseconds, so we don't need to do convert anymore.
switch to upstream kernel memfd implementation (ABI+API break)

Now that a generic memfd solution is merged upstream, drop our own
implementation and switch over.
test: adopt new memfd interface
Kconfig: depend on TMPFS
metadata: drop support for kernels < v3.15
Merge remote-tracking branch 'kdbus/3.17' into 3.16/kdbus
Revert "Merge branch '3.16/kdbus' into 3.16/my"

This reverts commit 90a796d011499b460cefc0ca4140c54f0d192450, reversing
changes made to 51e04c20c82fd5ab988ec27603c9fda865d799c1.
Merge branch '3.16/kdbus' into 3.16/my