Merge branch 'rely on ASSERT marcos in xdp_bpf2bpf.c/xdp_adjust_tail.c'

Lorenzo Bianconi says:

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Rely on ASSERT* macros and get rid of deprecated CHECK ones in xdp_bpf2bpf and
xdp_adjust_tail bpf selftests.
This is a preliminary series for XDP multi-frags support.

Changes since v1:
- run each ASSERT test separately
- drop unnecessary return statements
- drop unnecessary if condition in test_xdp_bpf2bpf()
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Signed-off-by: Andrii Nakryiko <andrii@kernel.org>