ABD style cleanups

The commit a6255b7fce400d485a0e87cbe369aa0ed7dc5dc4 removed a few
assertions which help catch errors and improve code readability. It also
duplicated two conditionals, which was unnecessary and made the code
confusing to read. This patch cleans it up.

Reviewed-by: David Quigley <david.quigley@intel.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Isaac Huang <he.huang@intel.com>
Closes #5802