filetop can't track file access behavior on sendfile system call

Hi all,

I use filetop to tracing behavior of access file, this tool use vfs_write() and vfs_read() to implement.

But when I use busybox (1.31.1) cat tool to output text on terminal, it default path is to use the do_sendfile() rule, not pass vfs_read() path, result in can't appear on filetop.

－busybox default [*] Use sendfile system call :
```
perf call-graph
22.74% 0.00% cat [kernel.kallsyms] [k] __sys_trace_return
|
---__sys_trace_return
|
|--8.22%--sys_sendfile64
| do_sendfile
| do_splice_direct
```

If I disable busybox setting sendfile system call, then cat process it will use vfs_read() path and will be displayed on filetop.

－[ ] Use sendfile system call :
```
perf call-graph
14.12% 14.12% cat [kernel.kallsyms] [k] generic_file_read_ite
|
---__sys_trace_return
sys_read
vfs_read
__vfs_read
```
So my question is whether the filetop can filter on sendfile() path, so that it can analysis more case for better user experience.