Merge branch 'bpf: Add detection of kfuncs.'

Alexei Starovoitov says:

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From: Alexei Starovoitov <ast@kernel.org>

Allow BPF programs detect at load time whether particular kfunc exists.

Patch 1: Allow ld_imm64 to point to kfunc in the kernel.
Patch 2: Fix relocation of kfunc in ld_imm64 insn when kfunc is in kernel module.
Patch 3: Introduce bpf_ksym_exists() macro.
Patch 4: selftest.

NOTE: detection of kfuncs from light skeleton is not supported yet.
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Signed-off-by: Andrii Nakryiko <andrii@kernel.org>