Fix problem with io_u being beyond EOF

Fix problem with io_u being beyond EOF

Bug report from "ljzhang,Yaxin Hu,Jianchao Tang" <nonggia@sjtu.edu.cn> :

1. The job file causing the problem:
----------ranmap-------------------
[global]
directory=./temp
nrfiles=1
rw=randread
size=64K
bsrange=1k-32k
thread
[ranmap]
description="Over-bounded in mark_random_map()."
-----------------------------------------

Running the job file, we have a high possibility to get a core dump like
this:
----------------------------------------
nonggia@nonggia-desktop:~/fio$ fio --version
fio 1.16.9
nonggia@nonggia-desktop:~/fio$ fio ranmap
ranmap: (g=0): rw=randread, bs=1K-32K/4K-4K, ioengine=sync, iodepth=1
Starting 1 thread
file:io_u.c:64, assert idx < f->num_maps failed
Segmentation fault (core dumped)
nonggia@nonggia-desktop:~/fio$ gdb fio core.6135
... ...
Core was generated by `fio ranmap'.
Program terminated with signal 11, Segmentation fault.
io_u.c:64
64                      fio_assert(td, idx < f->num_maps);
(gdb) bt
io_u.c:64
io_u.c:348
io_u.c:588
from /lib/tls/i686/cmov/libpthread.so.0
(gdb) p io_u->offset
$1 = 41984
(gdb) p io_u->buflen
$2 = 27648
(gdb) p io_u->file->real_file_size
$3 = 65536

2. Reason for the core dump:
The io_u->offset+io_u->buflen was too large to be map in the map file.
They just produced an idx out of the file_map's range.

The fix I produced made sure that get_next_buflen() doesn't go
beyond the real file size, by shrinking the buflen.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>