selftests/bpf: Add test for bpf_lsm_kernel_read_file()

Test the ability of bpf_lsm_kernel_read_file() to call the sleepable
functions bpf_ima_inode_hash() or bpf_ima_file_hash() to obtain a
measurement of a loaded IMA policy.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220302111404.193900-9-roberto.sassu@huawei.com