spa_export_common: refactor common exit points

Create a common exit point for spa_export_common (a very long
function), which avoids missing steps on failure. This work
is helpful for the planned forced pool export changes.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Will Andrews <will@firepipe.net>
Closes #11514