POWER: Add context-synchronizing instructions to pkey_write [BZ #25954]

POWER: Add context-synchronizing instructions to pkey_write [BZ #25954]

Sandipan Das reported that,

"The Power ISA mandates that all writes to the Authority
Mask Register (AMR) must always be preceded as well as
succeeded by a context-synchronizing instruction. This
applies to both the privileged and unprivileged variants
of the Move To AMR instruction.

This [requirement] is from Table 6 of Chapter 11 in page 1134 of Power
ISA 3.0B. The document can be found here:
<https://ibm.ent.box.com/s/1hzcwkwf8rbju5h9iyf44wm94amnlcrv>
"

See this kernel patch submission:

<https://lore.kernel.org/linuxppc-dev/5f65cf37be993760de8112a88da194e3ccbb2bf8.1588959697.git.sandipan@linux.ibm.com/>