x86/events, x86/insn-eval: Remove incorrect current->active_mm references

x86/events, x86/insn-eval: Remove incorrect current->active_mm references

When decoding an instruction or handling a perf event that references an
LDT segment, if we don't have a valid user context, trying to access the
LDT by any means other than SLDT is racy.  Certainly, using
current->active_mm is wrong, as active_mm can point to a real user mm when
CR3 and LDTR no longer reference that mm.

Clean up the code.  If nmi_uaccess_okay() says we don't have a valid
context, just fail.  Otherwise use current->mm.

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/20250402094540.3586683-3-mingo@kernel.org