selftests/bpf: Fix OOB read in dmabuf_collector

Dmabuf name allocations can be less than DMA_BUF_NAME_LEN characters,
but bpf_probe_read_kernel always tries to read exactly that many bytes.
If a name is less than DMA_BUF_NAME_LEN characters,
bpf_probe_read_kernel will read past the end. bpf_probe_read_kernel_str
stops at the first NUL terminator so use it instead, like
iter_dmabuf_for_each already does.

Fixes: ae5d2c59ecd7 ("selftests/bpf: Add test for dmabuf_iter")
Reported-by: Jerome Lee <jaewookl@quicinc.com>
Signed-off-by: T.J. Mercier <tjmercier@google.com>
Link: https://lore.kernel.org/r/20260225003349.113746-1-tjmercier@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>