selftests/bpf: Test writes to the buffer of bpf_dynptr_slice_rdwr()

bpf_dynptr_slice_rdwr() and bpf_dynptr_slice() can return the buffer. When
the buffer is on the stack, a store to the buffer invalidates the slice.
See "bpf: Invalidate a dynptr slice when its stack buffer is written".

dynptr_fail.c gets short progs with skb, xdp and file dynptrs. They cover
the events that invalidate a slice, the borders of the buffer, a copy of
the slice on the stack and in a caller, pruning, read-only slices, and
events that leave the slice valid. They end with a load through the slice
and are only loaded.

test_dynptr_skb_slice_rdwr_buffer in dynptr_success.c runs on a non-linear
skb. It shows that the slice is the buffer, and that the pattern of the
documentation continues to work. It passes without the fix too.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261102170958.3586893-3-alexei.starovoitov@gmail.com

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