fat: write: fix broken write to fragmented files

fat: write: fix broken write to fragmented files

The code for handing file overwrite incorrectly assumed that the file on
disk is always contiguous. This resulted in corrupting disk structure
every time when write to existing fragmented file happened. Fix this
by adding proper check for cluster discontinuity and adjust chunk size
on each partial write.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
This patch partially fixes the issue revealed by the following test
script:

--->8-fat_test1.sh---
#!/bin/bash
make sandbox_defconfig
make
dd if=/dev/zero of=/tmp/10M.img bs=1024 count=10k
mkfs.vfat -v /tmp/10M.img
cat >/tmp/cmds <<EOF
x
host bind 0 /tmp/10M.img
fatls host 0
mw 0x1000000 0x0a434241 0x1000 # "ABC\n"
mw 0x1100000 0x0a464544 0x8000 # "DEF\n"
fatwrite host 0 0x1000000 file0001.raw 0x1000
fatwrite host 0 0x1000000 file0002.raw 0x1000
fatwrite host 0 0x1000000 file0003.raw 0x1000
fatwrite host 0 0x1000000 file0004.raw 0x1000
fatwrite host 0 0x1000000 file0005.raw 0x1000
fatrm host 0 file0002.raw
fatrm host 0 file0004.raw
fatls host 0
fatwrite host 0 0x1100000 file0007.raw 0x4000
fatwrite host 0 0x1100000 file0007.raw 0x4000
reset
EOF
./u-boot </tmp/cmds
#verify
rm -r /tmp/result /tmp/model
mkdir /tmp/result
mkdir /tmp/model
yes ABC | head -c 4096 >/tmp/model/file0001.raw
yes ABC | head -c 4096 >/tmp/model/file0003.raw
yes ABC | head -c 4096 >/tmp/model/file0005.raw
yes DEF | head -c 16384 >/tmp/model/file0007.raw
mcopy -n -i /tmp/10M.img ::file0001.raw /tmp/result
mcopy -n -i /tmp/10M.img ::file0003.raw /tmp/result
mcopy -n -i /tmp/10M.img ::file0005.raw /tmp/result
mcopy -n -i /tmp/10M.img ::file0007.raw /tmp/result
hd /tmp/10M.img
if diff -urq /tmp/model /tmp/result
then
	echo Test okay
else
	echo Test fail
fi
--->8---

Overwritting a discontiguous test file (file0007.raw) no longer causes
corruption to file0003.raw, which's data lies between the chunks of the
test file. The amount of data written to disk is still incorrect, what
causes damage to the file (file0005.raw), which's data lies next to the
test file. This will be fixed by the next patch.

Feel free to prepare a proper sandbox/py_test based tests based on the
provided test scripts.