itch41: Avoid memcpy() in itch41_decode()

Remove the implicit memcpy() in itch41_decode() and let the caller
decide what to do with the message. This improves ITCH message decoding
performance up to 5%:

Before:

[penberg@golgotha libtrading]$ perf stat ./tools/test-itch41 ../data/nasdaq/S123110-v41.txt.gz
Decoding messages: 100%
 Performance counter stats for './tools/test-itch41 ../data/nasdaq/S123110-v41.txt.gz':

      18211.313992 task-clock                #    0.997 CPUs utilized
             8,450 context-switches          #    0.464 K/sec
                 0 cpu-migrations            #    0.000 K/sec
               572 page-faults               #    0.031 K/sec
    68,847,572,721 cycles                    #    3.780 GHz                     [83.33%]
    23,200,259,317 stalled-cycles-frontend   #   33.70% frontend cycles idle    [83.34%]
    15,248,176,165 stalled-cycles-backend    #   22.15% backend  cycles idle    [66.66%]
    88,276,934,750 instructions              #    1.28  insns per cycle
                                             #    0.26  stalled cycles per insn [83.34%]
    14,898,170,670 branches                  #  818.072 M/sec                   [83.32%]
     1,151,099,496 branch-misses             #    7.73% of all branches         [83.35%]

      18.265550473 seconds time elapsed

After:

[penberg@golgotha libtrading]$ perf stat ./tools/test-itch41 ../data/nasdaq/S123110-v41.txt.gz
Decoding messages: 100%
 Performance counter stats for './tools/test-itch41 ../data/nasdaq/S123110-v41.txt.gz':

      17345.070875 task-clock                #    0.997 CPUs utilized
             8,362 context-switches          #    0.482 K/sec
                 4 cpu-migrations            #    0.000 K/sec
               572 page-faults               #    0.033 K/sec
    65,569,032,443 cycles                    #    3.780 GHz                     [83.32%]
    24,233,187,125 stalled-cycles-frontend   #   36.96% frontend cycles idle    [83.34%]
    15,307,048,233 stalled-cycles-backend    #   23.34% backend  cycles idle    [66.67%]
    84,022,007,842 instructions              #    1.28  insns per cycle
                                             #    0.29  stalled cycles per insn [83.34%]
    13,990,083,749 branches                  #  806.574 M/sec                   [83.33%]
     1,036,831,608 branch-misses             #    7.41% of all branches         [83.34%]

      17.396997744 seconds time elapsed

Signed-off-by: Pekka Enberg <penberg@kernel.org>