x86, um: Mark system call tables readonly

Mark the system call tables readonly, as they already are on native,
and the 32-bit UM version was in the previous assembly version.  The
32-bit version lost it due to copy and paste from the 64-bit version,
which was missing the const.

Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Link: http://lkml.kernel.org/r/tip-45db1c6176c8171d9ae6fa6d82e07d115a5950ca@git.kernel.org
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/um/sys_call_table_32.c

@@ -43,7 +43,7 @@ typedef void (*sys_call_ptr_t)(void);
 
 extern void sys_ni_syscall(void);
 
-sys_call_ptr_t sys_call_table[] __cacheline_aligned = {
+const sys_call_ptr_t sys_call_table[] __cacheline_aligned = {
 	/*
 	 * Smells like a compiler bug -- it doesn't work
 	 * when the & below is removed.

arch/x86/um/sys_call_table_64.c

@@ -45,7 +45,7 @@ typedef void (*sys_call_ptr_t)(void);
 
 extern void sys_ni_syscall(void);
 
-sys_call_ptr_t sys_call_table[] __cacheline_aligned = {
+const sys_call_ptr_t sys_call_table[] __cacheline_aligned = {
 	/*
 	 * Smells like a compiler bug -- it doesn't work
 	 * when the & below is removed.