Fix ENXIO from spa_ld_verify_logs() in ztest

Fix ENXIO from spa_ld_verify_logs() in ztest

This patch fixes a small issue where the zil_check_log_chain()
code path would hit an EBUSY error. This would occur when
2 threads attempted to call metaslab_activate() at the same time.
In this case, the "loser" would receive an error code which should
have been ignored, but was instead floated to the caller. This
ended up resulting in an ENXIO being returned from from
spa_ld_verify_logs().

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8010