fio:pread() used buffer.IBUF_SHARED, which might be reused after a
yield. As a result, if pread() was called from 2 different fibers
or in parallel with something else using IBUF_SHARED, it would
turn the buffer into garbage for all parallel usages.

The same problem existed for read(), and was fixed in
c7c24f84 ("fio: Fix race condition
in fio.read"). But apparently pread() was missed.

What is worse, the original commit's test passed even without the
fix from that commit. Because it didn't check the results of
read()s called from 2 fibers.

The patch fixes pread() and adds a test covering both read() and
pread(). The old test from the original commit is dropped.

Follow up #3187