selftests/mm: fix build break when compiling pkey_util.c

selftests/mm: fix build break when compiling pkey_util.c

Commit 50910acd6f615 ("selftests/mm: use sys_pkey helpers consistently")
added a pkey_util.c to refactor some of the protection_keys functions
accessible by other tests.  But this broken the build in powerpc in two
ways,

pkey-powerpc.h: In function `arch_is_powervm':
pkey-powerpc.h:73:21: error: storage size of `buf' isn't known
   73 |         struct stat buf;
      |                     ^~~
pkey-powerpc.h:75:14: error: implicit declaration of function `stat'; did you mean `strcat'? [-Wimplicit-function-declaration]
   75 |         if ((stat("/sys/firmware/devicetree/base/ibm,partition-name", &buf) == 0) &&
      |              ^~~~
      |              strcat

Since pkey_util.c includes pkeys-helper.h, which in turn includes pkeys-powerpc.h,
stat.h including is missing for "struct stat". This is fixed by adding "sys/stat.h"
in pkeys-powerpc.h

Secondly,

pkey-powerpc.h:55:18: warning: format `%llx' expects argument of type `long long unsigned int', but argument 3 has type `u64' {aka `long unsigned int'} [-Wformat=]
   55 |         dprintf4("%s() changing %016llx to %016llx\n",
      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   56 |                          __func__, __read_pkey_reg(), pkey_reg);
      |                                    ~~~~~~~~~~~~~~~~~
      |                                    |
      |                                    u64 {aka long unsigned int}
pkey-helpers.h:63:32: note: in definition of macro `dprintf_level'
   63 |                 sigsafe_printf(args);           \
      |                                ^~~~

These format specifier related warning are removed by adding
"__SANE_USERSPACE_TYPES__" to pkeys_utils.c.

Link: https://lkml.kernel.org/r/20250428131937.641989-1-nysal@linux.ibm.com
Fixes: 50910acd6f61 ("selftests/mm: use sys_pkey helpers consistently")
Signed-off-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>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>