bpf: Fix doxygen comments for dynptr slice kfuncs

bpf: Fix doxygen comments for dynptr slice kfuncs

In commit 66e3a13e7c2c ("bpf: Add bpf_dynptr_slice and
bpf_dynptr_slice_rdwr"), the bpf_dynptr_slice() and
bpf_dynptr_slice_rdwr() kfuncs were added to BPF. These kfuncs included
doxygen headers, but unfortunately those headers are not properly
formatted according to [0], and causes the following warnings during the
docs build:

./kernel/bpf/helpers.c:2225: warning: \
    Excess function parameter 'returns' description in 'bpf_dynptr_slice'
./kernel/bpf/helpers.c:2303: warning: \
    Excess function parameter 'returns' description in 'bpf_dynptr_slice_rdwr'
...

This patch fixes those doxygen comments.

[0]: https://docs.kernel.org/doc-guide/kernel-doc.html#function-documentation

Fixes: 66e3a13e7c2c ("bpf: Add bpf_dynptr_slice and bpf_dynptr_slice_rdwr")
Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230301194910.602738-1-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>