selftests/bpf: Remove last bpf_create_map_xattr from test_verifier

bpf_create_map_xattr() call was reintroduced after merging bpf tree into
bpf-next tree. Convert the last instance into bpf_map_create() call.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211212191341.2529573-1-andrii@kernel.org