Assemble: keep MD_DISK_FAILFAST and MD_DISK_WRITEMOSTLY flag

Assemble: keep MD_DISK_FAILFAST and MD_DISK_WRITEMOSTLY flag

Before updating superblock of slave disks, desired_state value
is set for the target state of the slave disks. But it forgets
to check MD_DISK_FAILFAST and MD_DISK_WRITEMOSTLY flags. Then
start_arrays() calls ADD_NEW_DISK ioctl-call and pass the state
without MD_DISK_FAILFAST and MD_DISK_WRITEMOSTLY.

Currenlty it does not generate any problem because kernel does not
care MD_DISK_FAILFAST or MD_DISK_WRITEMOSTLY flags.

Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Gioh Kim <gi-oh.kim@profitbricks.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>