md/linear: fix hot-add of devices to linear arrays.

md/linear: fix hot-add of devices to linear arrays.

commit 789c30f2f706e8ae5e78cc7c41e18ebb6a16aa66
broke hot-add to a linear array.
After that commit, metadata if not written to devices until they
have been fully integrated into the array as determined by
saved_raid_disk.  That patch arranged to clear that field after
a recovery completed.

However for linear arrays, there is no recovery - the integration is
instantaneous.  So we need to explicitly clear the saved_raid_disk
field.

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