Fix various typos

Correct an assortment of typos throughout the code base.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Andrea Gelmini <andrea.gelmini@gelma.net>
Closes #11774

module/zfs/vdev_rebuild.c

@@ -81,7 +81,7 @@
  * Advantages:
  *
  *   - Sequential reconstruction is performed in LBA order which may be faster
- *     than healing reconstruction particularly when using using HDDs (or
+ *     than healing reconstruction particularly when using HDDs (or
  *     especially with SMR devices).  Only allocated capacity is resilvered.
  *
  *   - Sequential reconstruction is not constrained by ZFS block boundaries.