cpudist: Protect against potentially negative time deltas
cpudist: Protect against potentially negative time deltas

It seems from experimentation that the calculated timestamps between
on- and off-CPU switch events can produce incorrect results, with a
later event having a smaller timestamp. Discard events when the
resulting delta time would be negative.

tuxology/bcc,shodoco/bcc,iovisor/bcc,romain-intel/bcc,shodoco/bcc,iovisor/bcc,shodoco/bcc,brendangregg/bcc,tuxology/bcc,brendangregg/bcc,mcaleavya/bcc,mkacik/bcc,mkacik/bcc,iovisor/bcc,tuxology/bcc,romain-intel/bcc,brendangregg/bcc,shodoco/bcc,mkacik/bcc,romain-intel/bcc,tuxology/bcc,iovisor/bcc,brendangregg/bcc,romain-intel/bcc,shodoco/bcc,romain-intel/bcc,tuxology/bcc,mkacik/bcc,brendangregg/bcc,mcaleavya/bcc,mkacik/bcc,mcaleavya/bcc,mcaleavya/bcc,iovisor/bcc,mcaleavya/bcc