Fix zdb pool/ with -k

When examining the root dataset with zdb -k, we get into a mismatched
state. main() knows we are not examining the whole pool, but it strips
off the trailing slash. import_checkpointed_state() then thinks we are
examining the whole pool, and does not update the target path
appropriately. The fix is to directly inform import_checkpointed_state
that we are examining a filesystem, and not the whole pool.

Sponsored-by: Klara, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>
Signed-off-by: Paul Dagnelie <paul.dagnelie@klarasystems.com>
Co-authored-by: Paul Dagnelie <paul.dagnelie@klarasystems.com>
Closes #17536