sbd: add support for read/write cloned vdi

Tested-by: Marcin Mirosław <marcin@mejor.pl>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sbd: forbid to attach read-only snapshot

Tested-by: Marcin Mirosław <marcin@mejor.pl>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sbd: add support to compile against new kernel 3.12 and plus

Tested-by: Marcin Mirosław <marcin@mejor.pl>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sbd: make rq_for_each_segment happy for all the kernel version

Tested-by: Marcin Mirosław <marcin@mejor.pl>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
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,

$ 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.

Tested-by: Marcin Mirosław <marcin@mejor.pl>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sbd: don't enable sbd_debug as default

If dynamic debug enabled, sbd_debug() is enabled as default. This is not
necessary for plain users. So disable it default.

We can enable it dynamically by following command:

echo 'file sheep.c +p' > /sys/kernel/debug/dynamic_debug/control

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