md: be more consistent about setting WriteMostly flag when adding a drive to an array

md: be more consistent about setting WriteMostly flag when adding a drive to an array

When a drive is added to an array using ADD_NEW_DISK, there are two
places we can get certain flags from:  the metadata on the disk or the
flags passed through the IOCTL.

For the WriteMostly flag (aka MD_DISK_WRITEMOSTLY) we take the value
from either of those sources depending on if it is set (i.e. we
effectively 'or' the two sources together).

This makes it awkward to clear, and is at best inconsistent.

As documented code (in mdadm) requires that setting
MD_DISK_WRITEMOSTLY in the ioctl will be effective, we resolve the
inconsistency by always using the value for this flag from the ioctl,
and ignoring the value on disk.

Signed-off-by: NeilBrown <neilb@suse.de>