Improve pattern verfiy %o when thread=1 or verify_async > 1

[vincentkfu]

Currently fio does a malloc/free each time it verifies a block with a %o in its verify_pattern when thread=1 or verify_async > 1. This is slow and should be improved by doing the following:

• at init time determine how many buffers are needed and allocate them
• during verify, make each thread use its own buffer to avoid interfering with other threads

For more background see #1845