x86/entry: Avoid very early RET

x86/entry: Avoid very early RET

Commit

  0696ce348f2a ("x86/entry: Move PUSH_AND_CLEAR_REGS out of error_entry()")

manages to introduce a CALL/RET pair that is before SWITCH_TO_KERNEL_CR3,
which means it is before RETBleed can be mitigated.

Revert to an earlier version of the commit in Fixes. Down side is that
this will bloat .text size somewhat. The alternative is fully reverting
it.

The purpose of this patch was to allow migrating error_entry() to C,
including the whole of kPTI. Much care needs to be taken moving that
forward to not re-introduce this problem of early RETs.

Fixes: 0696ce348f2a ("x86/entry: Move PUSH_AND_CLEAR_REGS out of error_entry()")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>