Account for the fact that zpool clear can race error stat bumps.

tests/sys/cddl/zfs/include/libtest.kshlib
tests/sys/cddl/zfs/tests/cli_root/zpool_clear/zpool_clear_001_pos.ksh
zfs_ioc_clear resets error counts, but background processes may
re-increment them. For the purposes of this test, it only
fails if the test doesn't reset.

Submitted by: Will
Sponsored by: Spectra Logic Corp