Revert kthreads based on pthread thread specific data due to failure.

Revert kthreads based on pthread thread specific data due to failure.
With this patch applied I get the following failure 100% of the time,
I'd prefer to debug it and keep moving forward but I do not have the
time right now so I'm reverting the patch to the version which worked.
Ricardo please fix.

(gdb) bt
0  ztest_dmu_write_parallel (za=0x2aaaac898960) at
../../cmd/ztest/ztest.c:2566
1  0x0000000000405a79 in ztest_thread (arg=<value optimized out>)
    at ../../cmd/ztest/ztest.c:3862
2  0x00002b2e6a7a841d in zk_thread_helper (arg=<value optimized out>)
    at ../../lib/libzpool/kernel.c:131
3  0x000000379be06367 in start_thread (arg=<value optimized out>)
    at pthread_create.c:297
4  0x000000379b2d30ad in clone () from /lib64/libc.so.6