[PATCH] libata: make user scan wait for scan to complete

Make user scan wait for scan to complete.  This way user can wait for
warm plug request to complete and is prevented from causing EH event
storm by repetitively issuing scan request while EH is in progress.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Fajun Chen <fajunchen@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>