btrfs: add fstest for overwriting a file partially with RST

btrfs: add fstest for overwriting a file partially with RST

Add a test writing 128k to an empty file with one stripe already
pre-filled on-disk. Then overwrite a portion of the file in the middle.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
[Fixed the test statement and trailing white space in the .out file.]
Signed-off-by: Zorro Lang <zlang@kernel.org>