ec: make data strip as 512B to make VM and mkfs happy

1K is not suitable for some softwares like mkfs because it is operating on
sector size.

With sbd inserted,

# sbd0 mapped to an erasure coded vdi
$ mkfs.xfs -f /dev/sbd0 && xfs_check /dev/sbd0

will report error. Setting data strip as 512B solve the problem. This also
fixes the possible size problem in VM.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>