filesetup: fix size percentage calculations when using offset

filesetup: fix size percentage calculations when using offset

"When an fio job uses a non-zero offset value and a non-zero size
parameter, the working set size on disk is skewed. For a 64k file, the
job provided touches the following blocks:

dd if=/dev/zero of=/tmp/fiofile bs=64k count=1
fio --stonewall --filename=/tmp/fiofile --rw=write \
    --name=firstquarter  --size=25% --offset=0   --buffer_pattern=0x11 \
    --name=secondquarter --size=25% --offset=16k --buffer_pattern=0x22 \
    --name=thirdquarter  --size=25% --offset=32k --buffer_pattern=0x33

|1111111111111111|222222222222----|33333333--------|----------------|
0                16k              32k              48k              64k

1=written by firstquarter, 2=written by secondquarter, 3=written by
thirdquarter

It seems the expected vs. actual math on working set size is
(size_param_percent * size_of_file) vs.
(size_param_percent * (size_of_file - offset))."

Fix this by always calculating the percentage of the full file's size.
Also bound the I/O to be within the existing file's size when using a
size percentage to handle the case where the addition of an offset would
otherwise force I/O to go beyond the existing file size e.g.:

dd if=/dev/zero of=/tmp/fiofile bs=64k count=1
fio --stonewall --filename=/tmp/fiofile --rw=write \
    --name=beyond --size=100% --offset=32k --buffer_pattern=0x11

In the above, we ensure we only do 32k of I/O (rather than the 64k
implied by size=100%).

Fixes: https://github.com/axboe/fio/issues/377 ("Non-zero offset and
non-zero size skew disk working set size")

Reported-by: Brantley West <cbwest@gmail.com>
Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>