ZTS: Fix occasional inherit_001_pos.ksh failure

The mountpoint may still be busy when the zfs unmount -a command
is run causing an unexpected failure. Retry the unmount a couple
of times since it should not remain busy for long.

    19:10:50.29 NOTE: Reading state from .../inheritance/state021.cfg
    19:10:50.32 cannot unmount '/TESTPOOL': pool or dataset is busy
    19:10:50.32 ERROR: zfs unmount -a exited 1

Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13686