Handle closing an unopened ZVOL

Thank to commit a4430fce691d492aec382de0dfa937c05ee16500 we're
now correctly returning EROFS when opening a zvol on a read-only
pool. Unfortunately, it looks like this causes us to trigger
some unexpected behavior by __blkdev_get().

In the failure case it's possible blkdev_get() will call
blkdev_put() for a bdev which was never successfully opened.
This results in us trying to close the device again and hitting
the NULL dereference.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1343