CURRENT: kernel crash r326347: Cannot access memory at address 0x65657246

Since yesterday's CURRENT update, I face worse and nasty problems on
all of our CURRENT systems!

Most recent CURRENT, as with r326363, crashes on booting the kernel, in
another case the kernel is stuck and freezing after initialising the
USB subsystem (the last thing I see on screen, keyboard fully
functional, but no ACPI shutdown any more).

Another box is now two days in row after subsequent updates of CURRENT
multiple times a day crashing on load on ZFS filesystem. The last
known half-ways good kernel is 12.0-CURRENT #157 r326347: Wed Nov 29
01:02:47 CET 2017 amd64. When performing
buildworld/buildkernel/relaese/package or poudriere on a ZFS
filessystem, I can now bring down the system almost in a reproducable
way. 

Last error seen when core is dumped is:

/dev/stdin:1: Error in sourced command file:
Cannot access memory at address 0x65657246
/dev/stdin:1: Error in sourced command file:
Cannot access memory at address 0x65657246
/dev/stdin:1: Error in sourced command file:
Cannot access memory at address 0x65657246
/dev/stdin:1: Error in sourced command file:
Cannot access memory at address 0x65657246