selftests/bpf: improve error messages in veristat

Return error if preset parsing fails. Avoid proceeding with veristat run
if preset does not parse.
Before:
```
./veristat set_global_vars.bpf.o -G "arr[999999999999999999999] = 1"
Failed to parse value '999999999999999999999'
Processing 'set_global_vars.bpf.o'...
File                   Program           Verdict  Duration (us)  Insns  States  Program size  Jited size
---------------------  ----------------  -------  -------------  -----  ------  ------------  ----------
set_global_vars.bpf.o  test_set_globals  success             27     64       0            82           0
---------------------  ----------------  -------  -------------  -----  ------  ------------  ----------
Done. Processed 1 files, 0 programs. Skipped 1 files, 0 programs.
```
After:
```
./veristat set_global_vars.bpf.o -G "arr[999999999999999999999] = 1"
Failed to parse value '999999999999999999999'
Failed to parse global variable presets: arr[999999999999999999999] = 1
```

Improve error messages:
 * If preset struct member can't be found.
 * Array index out of bounds

Extract rtrim function.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250627144342.686896-1-mykyta.yatsenko5@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>