Fix intermittency in hotspare_import_001_pos

tests/sys/cddl/zfs/tests/hotspare/hotspare_import_001_pos.ksh
This test would occasionally fail because "zpool export" returns
EBUSY. That's not unexpected since it immediately follows "zpool
replace" and ZFS allows only one configuration change per
transaction group. Usually a txg will close as soon as resilvering
finishes, but sometimes "zpool export" sneaks in first. To fix the
problem, allow up to one retry of "zpool export"

Sponsored by: Spectra Logic Corp