x86/its: Fix build errors when CONFIG_MODULES=n

commit 9f35e33 upstream.

Fix several build errors when CONFIG_MODULES=n, including the following:

../arch/x86/kernel/alternative.c:195:25: error: incomplete definition of type 'struct module'
  195 |         for (int i = 0; i < mod->its_num_pages; i++) {

Fixes: 872df34 ("x86/its: Use dynamic thunks for indirect branches")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Dave Hansen <dave.hansen@intel.com>
Tested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[ pawan: backport: Bring ITS dynamic thunk code under CONFIG_MODULES ]
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 9f69fe3888f643b16c35c8583036e94c5322ca61)

Author: ebiggers

arch/x86/kernel/alternative.c

@@ -129,6 +129,7 @@ const unsigned char * const x86_nops[ASM_NOP_MAX+1] =
 
 #ifdef CONFIG_MITIGATION_ITS
 
+#ifdef CONFIG_MODULES
 static struct module *its_mod;
 static void *its_page;
 static unsigned int its_offset;
@@ -244,7 +245,16 @@ static void *its_allocate_thunk(int reg)
 	return thunk;
 }
 
-#endif
+#else /* CONFIG_MODULES */
+
+static void *its_allocate_thunk(int reg)
+{
+	return NULL;
+}
+
+#endif /* CONFIG_MODULES */
+
+#endif /* CONFIG_MITIGATION_ITS */
 
 /*
  * Fill the buffer with a single effective instruction of size @len.