x86/alternatives: adjust _apply_alternatives() to comply to Misra rule 18.2

While avoiding casts would be preferable here, the subtraction ends up
violating "Subtraction between pointers shall only be applied to pointers
that address elements of the same array". Use long arithmetic instead.

No functional change intended.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Dmytro Prokopchuk <dmytro_prokopchuk1@epam.com>

xen/arch/x86/alternative.c

@@ -342,13 +342,13 @@ static int init_or_livepatch _apply_alternatives(struct alt_instr *start,
 
         /* 0xe8/0xe9 are relative branches; fix the offset. */
         if ( a->repl_len >= 5 && (*buf & 0xfe) == 0xe8 )
-            *(int32_t *)(buf + 1) += repl - orig;
+            *(int32_t *)(buf + 1) += (long)repl - (long)orig;
         else if ( IS_ENABLED(CONFIG_RETURN_THUNK) &&
                   a->repl_len > 5 && buf[a->repl_len - 5] == 0xe9 &&
                   ((long)repl + a->repl_len +
                    *(int32_t *)(buf + a->repl_len - 4) ==
                    (long)__x86_return_thunk) )
-            *(int32_t *)(buf + a->repl_len - 4) += repl - orig;
+            *(int32_t *)(buf + a->repl_len - 4) += (long)repl - (long)orig;
 
         a->priv = 1;