SIGSEGV / Exit 139 when write_iolog used with io_submit_mode=offload

SIGSEGV / Exit 139 when write_iolog used with io_submit_mode=offload

Segmentation fault when log_io_u() attempts to write an entry to a
user-specified write_iolog file, if the I/O is issued from an offload
thread created by io_submit_mode=offload. Call path:

rate-submit.c::io_workqueue_fn() -> td_io_queue() -> log_io_u(td, io_u)

The log file handle in thread_data->iolog_f opened by init_iolog() is not
being copied to the offload thread's private copy of thread_data, causing a
NULL deference when fprintf() is called to write to the log file.

Fix is to copy the main thread's td->iolog_f to the offload thread's td at
creation time. Seems a bit disjointed to be copying individual fields between
these two structures on an as-needed basis rather than having a mechanism to
replicate the entire structure, or at least replicating the I/O submission
specific fields by moving them into a nested structure that's copied wholesale
in io_workqueue_init_worker_fn() - that way future code changes to the I/O
submission path wont cause the same bug for fields needed by both the inline
and offline submission paths.

Signed-off-by: Adam Horshack (horshack@live.com)