apps/examples/userfs/userfs_main.c: Fix warning (#41)

Fixes warning:

    CC:  userfs_main.c
    userfs_main.c:182:3: warning: initialization from incompatible pointer type [-Wincompatible-pointer-types]
       ufstest_truncate,
       ^
    userfs_main.c:182:3: note: (near initialization for ‘g_ufstest_ops.truncate’)

Co-authored-by: Gregory Nutt <gnutt@nuttx.org>

Author: patacongo

examples/userfs/userfs_main.c

@@ -115,7 +115,7 @@ static int     ufstest_dup(FAR void *volinfo, FAR void *oldinfo,
 static int     ufstest_fstat(FAR void *volinfo, FAR void *openinfo,
                  FAR struct stat *buf);
 static int     ufstest_truncate(FAR void *volinfo, FAR void *openinfo,
-                 off_t *length);
+                 off_t length);
 static int     ufstest_opendir(FAR void *volinfo, FAR const char *relpath,
                  FAR void **dir);
 static int     ufstest_closedir(FAR void *volinfo, FAR void *dir);
@@ -420,7 +420,7 @@ static int ufstest_fstat(FAR void *volinfo, FAR void *openinfo,
 }
 
 static int ufstest_truncate(FAR void *volinfo, FAR void *openinfo,
-                            off_t *length)
+                            off_t length)
 {
   return -ENOSYS;
 }