Do not relocate extents to make them contiguous if the underlying drive can do
deletions. Ability to do deletions is a strong indication that this
optimization will not help performance. It will only generate extra write
traffic. These devices are typically flash based and have a limited number of
write cycles. In addition, making the file contiguous in LBA space doesn't
improve the access times from flash devices because they have no seek time.

Reviewed by: mckusick@