x86/mm: Opt in to IRQs-off activate_mm()

We gain nothing by having the core code enable IRQs right before calling
activate_mm() only for us to turn them right back off again in switch_mm().

This will save a few cycles, so execve() should be blazingly fast with this
patch applied!

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/69c7d711f240cfec23e6024e940d31af2990db36.1641659630.git.luto@kernel.org