bpf: Don't take an extension for an iterator program

An extension inherits the expected_attach_type of the program it extends.
If that is BPF_TRACE_ITER, check_attach_btf_id() hands the extension to
bpf_iter_prog_supported(), which goes by the name of the attach function.
For an extension that is the name of the subprogram it replaces, so the
load fails with -EINVAL, unless that subprogram is called like the function
of a target, bpf_iter_bpf_map_elem for example.

Then the extension gets the context of that target.  If the subprogram
has as many arguments, it can use the key and the value as buffers, but
how far it reaches is noted in the extension, where the test of the
iterator link does not look: it reads and writes behind the elements of
the map.  And the id of the subprogram in the BTF of the program is cached
as the id of the target, so that programs of that target which are loaded
already cannot be attached.

Refuse all extensions here.

Fixes: 15d83c4d7cef ("bpf: Allow loading of a bpf_iter program")
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261017163745.3341787-27-alexei.starovoitov@gmail.com

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