Fix coverity defects: CID 186143

Fix coverity defects: CID 186143

CID 186143: Memory - illegal accesses (USE_AFTER_FREE)

This patch fixes an use-after-free in spa_import_progress_destroy()
moving the kmem_free() call at the end of the function.

Reviewed-by: Chris Dunlop <chris@onthe.net.au>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #8788