Use linear abd in vdev_copy_uberblocks()

Use linear abd in vdev_copy_uberblocks()

The vdev_copy_uberblocks() function should use abd_alloc_linear() to
allocate ub_abd, because abd_to_buf(ub_abd)) is used later.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Isaac Huang <he.huang@intel.com>
Closes #6718
Closes #6713