x86/entry: Fix entry_INT80_compat for Xen PV guests

Commit

  c89191ce ("x86/entry: Convert SWAPGS to swapgs and remove the definition of SWAPGS")

missed one use case of SWAPGS in entry_INT80_compat(). Removing of
the SWAPGS macro led to asm just using "swapgs", as it is accepting
instructions in capital letters, too.

This in turn leads to splats in Xen PV guests like:

  [   36.145223] general protection fault, maybe for address 0x2d: 0000 [#1] PREEMPT SMP NOPTI
  [   36.145794] CPU: 2 PID: 1847 Comm: ld-linux.so.2 Not tainted 5.19.1-1-default #1 \
	  openSUSE Tumbleweed f3b44bfb672cdb9f235aff53b57724eba8b9411b
  [   36.146608] Hardware name: HP ProLiant ML350p Gen8, BIOS P72 11/14/2013
  [   36.148126] RIP: e030:entry_INT80_compat+0x3/0xa3

Fix that by open coding this single instance of the SWAPGS macro.

Fixes: c89191ce ("x86/entry: Convert SWAPGS to swapgs and remove the definition of SWAPGS")
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Cc: <stable@vger.kernel.org> # 5.19
Link: https://lore.kernel.org/r/20220816071137.4893-1-jgross@suse.com

arch/x86/entry/entry_64_compat.S

@@ -311,7 +311,7 @@ SYM_CODE_START(entry_INT80_compat)
 	 * Interrupts are off on entry.
 	 */
 	ASM_CLAC			/* Do this early to minimize exposure */
-	SWAPGS
+	ALTERNATIVE "swapgs", "", X86_FEATURE_XENPV
 
 	/*
 	 * User tracing code (ptrace or signal handlers) might assume that