selftests/bpf: Mute expected invalid map creation error msg

Previously, the newly introduced test case in test_map_in_map(), which
checks whether the inner map is destroyed after unsuccessful creation of
the outer map, logged the following harmless and expected error:

    libbpf: map 'mim': failed to create: Invalid argument(-22) libbpf:
    failed to load object './test_map_in_map_invalid.o'

To avoid any possible confusion, mute the logging during loading of the
prog.

Fixes: 08f71a1e

 ("selftests/bpf: Check inner map deletion")
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Martynas Pumputis <m@lambda.lt>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210721140941.563175-1-m@lambda.lt