[librm] Do not unconditionally preserve flags across virt_call()

Commit 196f0f2 ("[librm] Convert prot_call() to a real-mode near
call") introduced a regression in which any deliberate modification to
the low 16 bits of the CPU flags (in struct i386_all_regs) would be
overwritten with the original flags value at the time of entry to
prot_call().

The regression arose because the alignment requirements of the
protected-mode stack necessitated the insertion of two bytes of
padding immediately below the prot_call() return address.  The
solution chosen was to extend the existing "pushfl / popfl" pair to
"pushfw;pushfl / popfl;popfw".  The extra "pushfw / popfw" appears at
first glance to be a no-op, but fails to take into account the fact
that the flags restored by popfl may have been deliberately modified
by the protected-mode function.

Fix by replacing "pushfw / popfw" with "pushw %ss / popw %ss".  While
%ss does appear within struct i386_all_regs, any modification to the
stored value has always been ignored by prot_call() anyway.

The most visible symptom of this regression was that SAN booting would
fail since every INT 13 call would be chained to the original INT 13
vector.

Reported-by: Vishvananda Ishaya <vishvananda@gmail.com>
Reported-by: Jamie Thompson <forum.ipxe@jamie-thompson.co.uk>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/transitions/librm.S

@@ -940,7 +940,7 @@ VC_OFFSET_END:
 	.globl virt_call
 virt_call:
 	/* Preserve registers, flags and GDT on external RM stack */
-	pushfw	/* padding */
+	pushw	%ss /* padding */
 	pushfl
 	pushal
 	pushw	%gs
@@ -1030,7 +1030,7 @@ vc_rmode:
 	 */
 	addr32 movl -20(%esp), %esp
 	popfl
-	popfw	/* padding */
+	popw	%ss /* padding */
 
 	/* Return and discard function parameters */
 	ret	$( VC_OFFSET_END - VC_OFFSET_PARAMS )