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

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>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/r/20250402094540.3586683-8-mingo@kernel.org