Remove lint ifdef checks in zdb and dbuf

This is effectively dead code for the Linux implementation which can
be removed to improve readability. We want to linter to check the
real production/debug build as much as possible.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #5722