ubi: tests: Speedup io_paral by using rand_r()

ubi: tests: Speedup io_paral by using rand_r()

rand() is not thread safe, but glibc seems to use a shared state which is
protected by a mutex. io_paral spawns a few threads and they call rand()
more or less in parallel, which causes heavy lock contention. That
makes the test extremely slow on some setups.

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: David Gstir <david@sigma-star.at>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>