opensnoop: error when trying to show/filter flags

I'm on Fedora 33, after running:
```
sudo /usr/share/bcc/tools/opensnoop -f O_WRONLY -f O_RDWR -e
```
I got:
```
bpf: Failed to load program: Permission denied
arg#0 type is not a struct
Unrecognized arg#0 type PTR
; KRETFUNC_PROBE(__x64_sys_openat2, struct pt_regs *regs, int ret)
0: (bf) r6 = r1
; KRETFUNC_PROBE(__x64_sys_openat2, struct pt_regs *regs, int ret)
1: (79) r9 = *(u64 *)(r6 +8)
2: (79) r1 = *(u64 *)(r6 +0)
func '__x64_sys_openat2' arg0 has btf_id 178 type STRUCT 'pt_regs'
; const char __user *filename = (char *)PT_REGS_PARM2(regs);
3: (79) r8 = *(u64 *)(r1 +104)
; struct open_how __user *how = (struct open_how *)PT_REGS_PARM3(regs);
4: (79) r1 = *(u64 *)(r1 +96)
; int flags = how->flags;
5: (79) r7 = *(u64 *)(r1 +0)
R1 invalid mem access 'inv'
processed 6 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
```
I got similar error when using only the `-e` flag.

## Fix
Now, I have no idea how all this works (yet...), so my fix below might not make sense, but intuitively, it seems to me that the pointer is dereferenced without being read, so I fixed it like this:
```patch
--- /tmp/opensnoop1 2020-12-13 13:07:21.415405454 +0100
+++ /tmp/opensnoop2 2020-12-13 13:08:29.936981179 +0100
@@ -215,8 +215,11 @@
{
int dfd = PT_REGS_PARM1(regs);
const char __user *filename = (char *)PT_REGS_PARM2(regs);
- struct open_how __user *how = (struct open_how *)PT_REGS_PARM3(regs);
- int flags = how->flags;
+ struct open_how __user how;
+ int flags;
+
+ bpf_probe_read_user(&how, sizeof(struct open_how), (struct open_how*)PT_REGS_PARM3(regs));
+ flags = how.flags;
#else
KRETFUNC_PROBE(FNNAME, int dfd, const char __user *filename, struct open_how __user *how, int ret)
{
```

Start with /usr/share/bcc/tools/opensnoop and the KRETFUNC_PROBE block for __x64_sys_openat2 shown in the report. Reproduce the Fedora 33 command using -f and -e, then verify that the openat2 probe loads and flag filtering works without the BPF verifier error.

Written by the indexing model from the issue text.