Since the upper regs don't exist for ia32 code, preserving them
doesn't hurt and it simplifies the code.

This doesn't add any attack surface that would not already be
available through INT80.

Notably:

 - 32bit SYSENTER: didn't clear si, dx, cx.

 - 32bit SYSCALL, INT80: *do* clear si since the C functions don't
   take a second argument.

 - 64bit: didn't clear si since the C functions take a second
   argument; except the error_entry path might have only one argument,
   so clearing si was missing here.

32b SYSENTER should be clearing all those 3 registers, nothing uses them
and selftests pass.

Unconditionally clear rsi since it simplifies code.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220506121631.293889636@infradead.org