Code cleanups

• Allocate ve_search on the stack, so we avoid allocating memory for every I/O even if the VDEV cache is disabled.
• Reduce lock scope.
• Avoid locking in vdev_cache_read() when the VDEV cache is disabled.
• Sort file names properly.
• Correct comment.

Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Pawel Jakub Dawidek <pawel@dawidek.net>
Closes #12749