shmfs/shmfs_alloc. Fix POSIX violation for shmfs_truncate

When shmfs_truncate is called, it uses shmfs_alloc_object to create the
physical backing for the shm file. However, the allocated physical
memory returned by mm_pgalloc is not cleared when CONFIG_BUILD_KERNEL is
set, which is a clear POSIX violation:

https://pubs.opengroup.org/onlinepubs/9699919799/functions/truncate.html

"If the file was previously shorter than length, its size is increased,
and the extended area appears as if it were zero-filled."

For FLAT and PROTECTED modes zalloc is used, so the violation affects
KERNEL mode only.

fs/shm/shmfs_alloc.c

@@ -22,7 +22,11 @@
  * Included Files
  ****************************************************************************/
 
+#include <nuttx/config.h>
+
 #include <stdbool.h>
+
+#include <nuttx/arch.h>
 #include <nuttx/kmalloc.h>
 #include <nuttx/pgalloc.h>
 
@@ -87,6 +91,12 @@ FAR struct shmfs_object_s *shmfs_alloc_object(size_t length)
             {
               break;
             }
+          else
+            {
+              /* Clear the page memory (requirement for truncate) */
+
+              up_addrenv_page_wipe((uintptr_t)pages[i]);
+            }
         }
     }