selftests/mm: fix a build failure on powerpc

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: 46036188ea1f ("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>