x86/nospec: Fix i386 RSB stuffing

x86/nospec: Fix i386 RSB stuffing

Origin: https://git.kernel.org/linus/332924973725e8cdcc783c175f68cf7e162cb9e5
Bug-Debian: https://bugs.debian.org/1017425

Turns out that i386 doesn't unconditionally have LFENCE, as such the
loop in __FILL_RETURN_BUFFER isn't actually speculation safe on such
chips.

Fixes: ba6e31af2be9 ("x86/speculation: Add LFENCE to RSB fill sequence")
Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/Yv9tj9vbQ9nNlXoY@worktop.programming.kicks-ass.net
Gbp-Pq: Topic bugfix/x86
Gbp-Pq: Name x86-nospec-Fix-i386-RSB-stuffing.patch