sparc64: Do not save/restore interrupts in get_new_mmu_context()

get_new_mmu_context() is always called with interrupts disabled.
So it's possible to do this micro optimization.

(Also fix the comment to switch_mm, which is called in both cases)

Signed-off-by: Kirill Tkhai <tkhai@yandex.ru>
CC: David Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>