x86/alternatives: fix .init section reference in _apply_alternatives()

The code in _apply_alternatives() will unconditionally attempt to read
__initdata_cf_clobber_{start,end} when called as part of applying alternatives
to a livepatch payload when Xen is using IBT.

That leads to a page-fault as __initdata_cf_clobber_{start,end} living in
.init section will have been unmapped by the time a livepatch gets loaded.

Fix by adding a check that limits the clobbering of endbr64 instructions to
boot time only.

Fixes: 37ed5da851 ('x86/altcall: Optimise away endbr64 instruction where possible')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
(cherry picked from commit 4be1fef1e6)

xen/arch/x86/alternative.c

@@ -338,7 +338,7 @@ static void init_or_livepatch _apply_alternatives(struct alt_instr *start,
      * Clobber endbr64 instructions now that altcall has finished optimising
      * all indirect branches to direct ones.
      */
-    if ( force && cpu_has_xen_ibt )
+    if ( force && cpu_has_xen_ibt && system_state < SYS_STATE_active )
     {
         void *const *val;
         unsigned int clobbered = 0;