selftests/bpf: Fix bpf selftest build error

selftests/bpf: Fix bpf selftest build error

On linux-next, build for bpf selftest displays an error due to
mismatch in the expected function signature of bpf_testmod_test_read
and bpf_testmod_test_write.

Commit 97d06802d10a ("sysfs: constify bin_attribute argument of bin_attribute::read/write()")
changed the required type for struct bin_attribute to const struct bin_attribute.

To resolve the error, update corresponding signature for the callback.

Fixes: 97d06802d10a ("sysfs: constify bin_attribute argument of bin_attribute::read/write()")
Reported-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Closes: https://lore.kernel.org/all/e915da49-2b9a-4c4c-a34f-877f378129f6@linux.ibm.com/
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
Link: https://lore.kernel.org/r/20250512091108.2015615-1-skb99@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>