Prevent panic during concurrent snapshot rollback and zvol read

Protect zvol_cdev_read with zv_suspend_lock to prevent concurrent
release of the dnode, avoiding panic when a snapshot is rolled back
in parallel during ongoing zvol read operation.

Reviewed-by: Chunwei Chen <tuxoko@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #14839