Fix preadv203 for fast storage (NVME)

[pevik]

preadv203 fails when storage is fast:

preadv203.c:145: TINFO: Number of full_reads 2920600, short reads 0, zero len reads 0, EAGAIN(s) 0
preadv203.c:180: TINFO: Number of writes 3535680
preadv203.c:225: TFAIL: Haven't got EAGAIN

Test pass a flag to the pread2() syscall not to wait for storage and return without any data in the case that the call would have to sleep in kernel. Then it tries to saturate the I/O so that some of the requests return will return without data.

However the test currently does not scale that well. On very fast NVME it tends to fail like this. We have to either figure out how to saturate the bus better or change it not to report a failures on fast devices.