btrfs: add fstest for writing to a file at an offset with RST

btrfs: add fstest for writing to a file at an offset with RST

Add a fstest writing 4k at offset 64k to a file with one RAID tripe
already pre-filled for a raid-stripe-tree formatted file system.

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>