selftests/bpf: add positive tests for new VFS based BPF kfuncs

Add a bunch of positive selftests which extensively cover the various
contexts and parameters in which the new VFS based BPF kfuncs may be
used from.

Again, the following VFS based BPF kfuncs are thoroughly tested within
this new selftest:
* struct file *bpf_get_task_exe_file(struct task_struct *);
* void bpf_put_file(struct file *);
* int bpf_path_d_path(struct path *, char *, size_t);

Acked-by: Christian Brauner <brauner@kernel.org>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
Link: https://lore.kernel.org/r/20240731110833.1834742-4-mattbobrowski@google.com


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