Introduce the data_input map, write-protected with a small eBPF program
implementing the lsm/bpf_map hook.

Then, ensure that bpf_map_get_fd_by_id() and bpf_map_get_fd_by_id_opts()
with NULL opts don't succeed due to requesting read-write access to the
write-protected map. Also, ensure that bpf_map_get_fd_by_id_opts() with
open_flags in opts set to BPF_F_RDONLY instead succeeds.

After obtaining a read-only fd, ensure that only map lookup succeeds and
not update. Ensure that update works only with the read-write fd obtained
at program loading time, when the write protection was not yet enabled.

Finally, ensure that the other _opts variants of bpf_*_get_fd_by_id() don't
work if the BPF_F_RDONLY flag is set in opts (due to the kernel not
handling the open_flags member of bpf_attr).
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221006110736.84253-7-roberto.sassu@huaweicloud.com