bpf: improve verification speed by not remarking live_read

With large verifier speed improvement brought by the previous patch
mark_reg_read() becomes the hottest function during verification.
On a typical program it consumes 40% of cpu.
mark_reg_read() walks parentage chain of registers to mark parents as LIVE_READ.
Once the register is marked there is no need to remark it again in the future.
Hence stop walking the chain once first LIVE_READ is seen.
This optimization drops mark_reg_read() time from 40% of cpu to <1%
and overall 2x improvement of verification speed.
For some programs the longest_mark_read_walk counter improves from ~500 to ~5

Signed-off-by: Alexei Starovoitov <[email protected]>
Reviewed-by: Jakub Kicinski <[email protected]>
Reviewed-by: Edward Cree <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>

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