dm raid: fix stripe adding reshape deadlock

When initiating a stripe adding reshape, a deadlock between
md_stop_writes() waiting for the sync thread to stop and the running
sync thread waiting for inactive stripes occurs (this frequently happens
on single-core but rarely on multi-core systems).

Fix this deadlock by setting MD_RECOVERY_WAIT to have the main MD
resynchronization thread worker (md_do_sync()) bail out when initiating
the reshape via constructor arguments.

Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>