Merge branch 'libbpf: add bpf_core_cast() helper'

Andrii Nakryiko says:

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Add bpf_core_cast(<ptr>, <type>) macro wrapper around bpf_rdonly_cast() kfunc
to make it easier to use this functionality in BPF code. See patch #2 for
BPF selftests conversions demonstrating improvements in code succinctness.
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Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>