filesetup: Initialize all members of struct fio_file

This patch avoids that Valgrind reports complaints like the following:

==10492== Conditional jump or move depends on uninitialised value(s)
==10492==    at 0x43BF19: fio_file_reset (filesetup.c:1872)
==10492==    by 0x43AB7F: alloc_new_file (filesetup.c:1496)
==10492==    by 0x43AED4: add_file (filesetup.c:1581)
==10492==    by 0x4528D3: str_filename_cb (options.c:1276)
==10492==    by 0x44D83A: __handle_option (parse.c:722)
==10492==    by 0x44E303: handle_option (parse.c:941)
==10492==    by 0x44E68B: parse_cmd_option (parse.c:1061)
==10492==    by 0x4541DD: fio_cmd_option_parse (options.c:4966)
==10492==    by 0x427C6D: parse_cmd_line (init.c:2652)
==10492==    by 0x428685: parse_options (init.c:2924)
==10492==    by 0x4920AC: main (fio.c:47)

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>