ZTS: speed up rsend tests

With some minor tweaks several of rsend tests can be sped up
considerably without significantly reducing test coverage.

• send-c_verify_ratio: ~120s -> ~60s
• send_realloc_*_files: ~330s -> ~65s

For the send_realloc* tests this also has the advantage of removing
(most of) the linux/freebsd conditional logic. Note that for this
test more passes, and thus more incremental send/recvs, are preferable
to a larger number of files.

Total run time of the rsend test group was reduced from roughly 20 to
11 minutes in an environment similar to what's used by the CI.

Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12876