io_uring: Replace pthread_self with s->tid

io_uring: Replace pthread_self with s->tid

__init_rand64 takes 64bit value and srand48 takes unsigned 32bit value,
pthread_t is opaque type and some libcs ( e.g. musl ) do not define them
in plain old data types and ends up with errors

| t/io_uring.c:809:32: error: incompatible pointer to integer conversion passing 'pthread_t' (aka 'struct __pthread *') to parameter of type 'uint64_t' (aka 'unsigned long') [-Wint-conver
sion]
|         __init_rand64(&s->rand_state, pthread_self());
|                                       ^~~~~~~~~~~~~~

Signed-off-by: Khem Raj <raj.khem@gmail.com>