Add fsmark module

fsmark is an I/O benchmark tool.
fsmark originally uses fork when kicking off new tasks, so I had to change
the code to use threads instead.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
[ tgrabiec: resolved minor conflict in modules.json ]
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>