selftests/mm: fix a build failure on powerpc

The compiler is unaware of the size of code generated by the ".rept"
assembler directive.  This results in the compiler emitting branch
instructions where the offset to branch to exceeds the maximum allowed
value, resulting in build failures like the following:

  CC       protection_keys
  /tmp/ccypKWAE.s: Assembler messages:
  /tmp/ccypKWAE.s:2073: Error: operand out of range (0x0000000000020158
  is not between 0xffffffffffff8000 and 0x0000000000007ffc)
  /tmp/ccypKWAE.s:2509: Error: operand out of range (0x0000000000020130
  is not between 0xffffffffffff8000 and 0x0000000000007ffc)

Fix the issue by manually adding nop instructions using the preprocessor.

Link: https://lkml.kernel.org/r/20250428131937.641989-2-nysal@linux.ibm.com
Fixes: 4603618 ("selftests/mm: build with -O2")
Reported-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Signed-off-by: Nysal Jan K.A. <nysal@linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Reviewed-by: Donet Tom <donettom@linux.ibm.com>
Tested-by: Donet Tom <donettom@linux.ibm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Nysal Jan K.A.

tools/testing/selftests/mm/pkey-powerpc.h

@@ -104,8 +104,18 @@ static inline void expect_fault_on_read_execonly_key(void *p1, int pkey)
 	return;
 }
 
+#define REPEAT_8(s) s s s s s s s s
+#define REPEAT_64(s) REPEAT_8(s) REPEAT_8(s) REPEAT_8(s) REPEAT_8(s) \
+		     REPEAT_8(s) REPEAT_8(s) REPEAT_8(s) REPEAT_8(s)
+#define REPEAT_512(s) REPEAT_64(s) REPEAT_64(s) REPEAT_64(s) REPEAT_64(s) \
+		      REPEAT_64(s) REPEAT_64(s) REPEAT_64(s) REPEAT_64(s)
+#define REPEAT_4096(s) REPEAT_512(s) REPEAT_512(s) REPEAT_512(s) REPEAT_512(s) \
+		       REPEAT_512(s) REPEAT_512(s) REPEAT_512(s) REPEAT_512(s)
+#define REPEAT_16384(s) REPEAT_4096(s) REPEAT_4096(s) \
+			REPEAT_4096(s) REPEAT_4096(s)
+
 /* 4-byte instructions * 16384 = 64K page */
-#define __page_o_noops() asm(".rept 16384 ; nop; .endr")
+#define __page_o_noops() asm(REPEAT_16384("nop\n"))
 
 static inline void *malloc_pkey_with_mprotect_subpage(long size, int prot, u16 pkey)
 {