Fix occasional rsend test crashes

We have occasional crashes in the rsend tests. Debugging revealed
that this is because the send_worker thread is getting EINTR from
splice(). This happens when a non-fatal signal is received during
the syscall. We should retry the syscall, rather than exiting failure.
Tweak the loop to only break if the splice is finished or we receive
a non-EINTR error.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #15273