KTLS with zfs recv

[Alan Somers]

My understanding is that KTLS works very well with OpenSSL for sending, but
not as well for receiving, because there's nothing like a recvfile
syscall.  However, it works great for both send and receive with NFS, where
all the data remains in the kernel. What about zfs recv?  A very common
pattern is for an application to read from an SSL socket and then pipe the
data to zfs recv. For example, zrepl does that.  Could zfs recv instead
read directly from the KTLS socket, bypassing userspace?  That could
potentially save a _lot_ of cycles for a _lot_ of people.

-Alan