bpf: Size backward jumps by the insns they jump over

A backward jump is sized from its target's current-pass address and its
own previous-pass address, so it can grow while other jumps shrink. Two
passes may then give the same image length with jumps still changing,
which ends in "bpf_jit: fatal error", and a jump growing in a padding
pass ends in "unexpected jmp_cond padding".

Shift the jump's previous-pass address by how far the current pass has
moved it, so a backward jump is sized by the insns it jumps over, like
a forward one. Also start every insn at BPF_MAX_INSN_SIZE instead of 64,
so none can grow in the first pass. Then no insn grows from one pass to
the next, and a repeated image length means nothing moved. The 123
limit on imm8 offsets worked around that growth and is dropped. The
ja -1 special case is subsumed, such a jump is now imm8 from the first
pass.

Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://patch.msgid.link/20261022-jit_size_fix-v2-1-4bee6921b218@meta.com

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