x86/spec_ctrl: Fix several bugs in SPEC_CTRL_ENTRY_FROM_INTR_IST

x86/spec_ctrl: Fix several bugs in SPEC_CTRL_ENTRY_FROM_INTR_IST

DO_OVERWRITE_RSB clobbers %rax, meaning in practice that the bti_ist_info
field gets zeroed.  Older versions of this code had the DO_OVERWRITE_RSB
register selectable, so reintroduce this ability and use it to cause the
INTR_IST path to use %rdx instead.

The use of %dl for the %cs.rpl check means that when an IST interrupt hits
Xen, we try to load 1 into the high 32 bits of MSR_SPEC_CTRL, suffering a #GP
fault instead.

Also, drop an unused label which was a copy/paste mistake.

Reported-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reported-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>