Fix use after free regression in spa_remove_healed_errors()

6839ec6f1098c28ff7b772f1b31b832d05e6b567 placed code in
spa_remove_healed_errors() that uses a pointer after the kmem_free()
call that frees it.

Reported-by: Coverity (CID-1562375)
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Amanakis <gamanakis@gmail.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14860