Fix race in vdev_initialize_thread

Before allowing new allocations to the metaslab we need to ensure
that any issued initializing writes have been synced. Otherwise,
it's possible for metaslab_block_alloc() to allocate a range which
is about to be overwritten by an initializing IO.

Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Tim Chase <tim@chase2k.com>
Reviewed-by: George Wilson <george.wilson@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8461