Get rid of find_rdev_all

find_rdev_all is now only used to check if a device is already
used in an md array.

We change lock_rdev so that it claims the bdev for
the specific rdev rather than for rdevs in general.
Now lock_rdev will check if the bdev is inuse by another array
or not, so the find_rdev_all check isn't needed and is removed,
along with find_rdev_all itself.

We also make sure that the error code from lock_rdev is
propagated up properly.