Fix indefinite article

The indefinite article before nvlist should be "an", not "a".

We have 27 "an nvlist" and 7 "a nvlist" in our comment, they should
stay the same as we are such a strict filesystem.

Signed-off-by: GeLiXin <ge.lixin@zte.com.cn>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4941