x86/CPU: Add a tlbi= cmdline switch

x86/CPU: Add a tlbi= cmdline switch

With the recently found INVLPGB / TLBSYNC issue, there has been some
interest in disabling INVLPGB-based TLB flushing, in order to rule out
that CPU issue as a cause of userspace crashes.

Add a kernel command line option to control the TLB flushing behavior.

If the need arises, we will add a "tlbi=broadcast" for the case when TLB
invalidation broadcasts need to be explicitly selected, but this is not
needed now yet.

  [ bp: Rewrite commit message, move to cpu/common.c, add documentation. ]

Fixes: 767ae437a32d ("x86/mm: Add INVLPGB feature and Kconfig entry")
Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: <stable@kernel.org>
Link: https://patch.msgid.link/20260729204341.3eb0b5ea@fangorn