tools: Fix compilation error due to rewriter update

Since ad2d0d9f, the bcc rewriter is able to track more external
pointers going through maps.  xfsslower and zfsslower were relying on
the rewriter not being able to replace some dereferences.  This
commit takes this into account and removes two unnecessary calls to
bpf_probe_read.

tools/xfsslower.py

@@ -187,10 +187,7 @@ static int trace_return(struct pt_regs *ctx, int type)
     bpf_get_current_comm(&data.task, sizeof(data.task));
 
     // workaround (rewriter should handle file to d_name in one step):
-    struct dentry *de = NULL;
-    struct qstr qs = {};
-    bpf_probe_read(&de, sizeof(de), &valp->fp->f_path.dentry);
-    bpf_probe_read(&qs, sizeof(qs), (void *)&de->d_name);
+    struct qstr qs = valp->fp->f_path.dentry->d_name;
     if (qs.len == 0)
         return 0;
     bpf_probe_read(&data.file, sizeof(data.file), (void *)qs.name);

tools/zfsslower.py

@@ -190,11 +190,7 @@ static int trace_return(struct pt_regs *ctx, int type)
     data.offset = valp->offset;
     bpf_get_current_comm(&data.task, sizeof(data.task));
 
-    // workaround (rewriter should handle file to d_name in one step):
-    struct dentry *de = NULL;
-    struct qstr qs = {};
-    bpf_probe_read(&de, sizeof(de), &valp->fp->f_path.dentry);
-    bpf_probe_read(&qs, sizeof(qs), (void *)&de->d_name);
+    struct qstr qs = valp->fp->f_path.dentry->d_name;
     if (qs.len == 0)
         return 0;
     bpf_probe_read(&data.file, sizeof(data.file), (void *)qs.name);