Fix various Coverity-detected errors in nvmecontrol

This fixes several Coverity-detected errors in nvmecontrol. While in
here, a couple additional errors with shift/mask confusion that were
not diagnosed by Coverity are also fixed.

CIDs addressed: 1040299, 1040300, 1403972, 1403973, 1403985, 1403988,
1403990, 1404374, 1404427, 1404469, 1404510, 1404534, 1418118

CID 1403657 (resource leak of shared library handle) was marked
"intentional" in the Coverity scan database.

Reviewed by: vangyzen, robert.herndon_dell.com
Reviewed by: daniel.william.ryan_gmail.com (earlier version)
Reviewed by: rramsden_isilon.com (earlier version), imp
MFC after: 5 days
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D24203