Fix coverity defects: CID 184285

CID 184285: Read from pointer after free (USE_AFTER_FREE)

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

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #8120