kvm: Directly account vtime to system on guest switch

Switching to or from guest context is done on ioctl context.
So by the time we call kvm_guest_enter() or kvm_guest_exit()
we know we are not running the idle task.

As a result, we can directly account the cputime using
vtime_account_system().

There are two good reasons to do this:

* We avoid some useless checks on guest switch. It optimizes
a bit this fast path.

* In the case of CONFIG_IRQ_TIME_ACCOUNTING, calling vtime_account()
checks for irq time to account. This is pointless since we know
we are not in an irq on guest switch. This is wasting cpu cycles
for no good reason. vtime_account_system() OTOH is a no-op in
this config option.

* We can remove the irq disable/enable around kvm guest switch in s390.

A further optimization may consist in introducing a vtime_account_guest()
that directly calls account_guest_time().

Signed-off-by: Frederic Weisbecker <[email protected]>
Cc: Tony Luck <[email protected]>
Cc: Fenghua Yu <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Cc: Avi Kivity <[email protected]>
Cc: Marcelo Tosatti <[email protected]>
Cc: Joerg Roedel <[email protected]>
Cc: Alexander Graf <[email protected]>
Cc: Xiantao Zhang <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Cornelia Huck <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Paul Gortmaker <[email protected]>

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