[SRU][Patch 1/2][Bionic 4.15] Hot adding a core with drmgr

[Manoj Iyer]

Please consider this SRU request for Bionic 4.15 kernel on PowerPC for
bug: https://bugs.launchpad.net/bugs/1833716, where hot adding CPUs causes 
the kernel to crash.

The patches were cleanly cherrypicked from upstream linus tree and
tested by me on Power8 LPAR, and found to fix this issue. Please refer
to the bug report for details on testing. 

Thanks
Manoj Iyer