[IA64] Don't alloc empty frame in ia64_switch_mode_phys

I think ia64_switch_mode_phys and ia64_switch_mode_virt
does not need to alloc an empty frame.
An empty frame is required by loadrs but flushrs
does not need that.
Signed-off-by: Zou Nan hai <nanhai.zou@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>

arch/ia64/kernel/head.S

@@ -853,7 +853,6 @@ END(__ia64_init_fpu)
  */
 GLOBAL_ENTRY(ia64_switch_mode_phys)
  {
-	alloc r2=ar.pfs,0,0,0,0
 	rsm psr.i | psr.ic		// disable interrupts and interrupt collection
 	mov r15=ip
  }
@@ -902,7 +901,6 @@ END(ia64_switch_mode_phys)
  */
 GLOBAL_ENTRY(ia64_switch_mode_virt)
  {
-	alloc r2=ar.pfs,0,0,0,0
 	rsm psr.i | psr.ic		// disable interrupts and interrupt collection
 	mov r15=ip
  }