sys/linux: support callback BPF functions

Commit [1] upstream introduced a new BPF helper, bpf_for_each_map_elem,
which comes with callback functions (BPF programs). The callback
function's address is provided via a 64-bit IMM load instruction where
the first 32-bit of the immediate value are the offset from the current
instruction to the start of the callback function. The 64-bit value is
then rewritten into the address of the callback function.

Callback BPF functions are similar to BPF_PSEUDO_CALL functions, except
the offset to the function is converted into an address to be passed to
a BPF helper. The same workaround is thus used to encode the offset in
syzkaller, given we can't currently limit the offset to the program
size.

1 - https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=69c087ba6225b
Signed-off-by: Paul Chaignon <paul@isovalent.com>

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