Move dmu_buf_rele() after dsl_dataset_sync_done()

Otherwise the dataset may be freed after the last dmu_buf_rele() leading
to a panic.

Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #14522
Closes #14523