x86/xstate: Elide redundant writes in set_xcr0()

x86/xstate: Elide redundant writes in set_xcr0()

XSETBV is an expensive instruction as, amongst other things, it involves
reconfiguring the instruction decode at the frontend of the pipeline.

We have several paths which reconfigure %xcr0 in quick succession (the context
switch path has 5, including the fpu save/restore helpers), and only a single
caller takes any care to try to skip redundant writes.

Update set_xcr0() to perform amortisation automatically, and simplify the
__context_switch() path as a consequence.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>