defconfig: Disable mutex debugging

Disabling mutex debugging results in 10.1% advantage in hackbench (10 runs).

Pre:
 Performance counter stats for 'hackbench 10  ' (10 runs):

       3384.850206 task-clock                #    3.637 CPUs utilized
 ( +-  0.76% )
             13433 context-switches          #    0.004 M/sec
 ( +-  8.93% )
              3569 CPU-migrations            #    0.001 M/sec
 ( +-  8.01% )
             21536 page-faults               #    0.006 M/sec
 ( +-  0.01% )
        6975089407 cycles                    #    2.061 GHz
 ( +-  0.82% ) [87.54%]
                 0 stalled-cycles-frontend   #    0.00% frontend cycles idle
 ( +-  0.00% ) [86.32%]
                 0 stalled-cycles-backend    #    0.00% backend  cycles idle
 ( +-  0.00% ) [86.60%]
        2142191803 instructions              #    0.31  insns per cycle
 ( +-  0.69% ) [87.60%]
         205066125 branches                  #   60.584 M/sec
 ( +-  0.40% ) [88.45%]
           5638437 branch-misses             #    2.75% of all branches
 ( +-  2.94% ) [78.00%]

       0.930762612 seconds time elapsed
 ( +-  1.37% )

Post:
 Performance counter stats for 'hackbench 10  ' (10 runs):

       2996.015588 task-clock                #    3.582 CPUs utilized
 ( +-  1.08% )
             17953 context-switches          #    0.006 M/sec
 ( +- 12.73% )
              4908 CPU-migrations            #    0.002 M/sec
 ( +- 11.00% )
             21536 page-faults               #    0.007 M/sec
 ( +-  0.00% )
        6149072628 cycles                    #    2.052 GHz
 ( +-  1.40% ) [86.50%]
                 0 stalled-cycles-frontend   #    0.00% frontend cycles idle
 ( +-  0.00% ) [86.23%]
                 0 stalled-cycles-backend    #    0.00% backend  cycles idle
 ( +-  0.00% ) [88.41%]
        2026544975 instructions              #    0.33  insns per cycle
 ( +-  0.54% ) [89.85%]
         197011679 branches                  #   65.758 M/sec
 ( +-  0.51% ) [89.98%]
           4655940 branch-misses             #    2.36% of all branches
 ( +-  2.94% ) [77.82%]

       0.836359125 seconds time elapsed
 ( +-  1.99% )