context_tracking: avoid irq_save/irq_restore on guest entry and exit

guest_enter and guest_exit must be called with interrupts disabled,
since they take the vtime_seqlock with write_seq{lock,unlock}.
Therefore, it is not necessary to check for exceptions, nor to
save/restore the IRQ state, when context tracking functions are
called by guest_enter and guest_exit.

Split the body of context_tracking_entry and context_tracking_exit
out to __-prefixed functions, and use them from KVM.

Rik van Riel has measured this to speed up a tight vmentry/vmexit
loop by about 2%.

Cc: Andy Lutomirski <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Paul McKenney <[email protected]>
Reviewed-by: Rik van Riel <[email protected]>
Tested-by: Rik van Riel <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

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