sendfile compat checks shouldn't be done in so_sendmblk
From the original Joyent bug:
Currently we're checking in so_sendmblk whether or not we are using sendfile with this interface and whether the underlying protocol supports it. However, it's actually being done at the wrong layer in essence. One can use the so_sendmblk interface, such as with ksockets, without needing to use sendfile at all. And in fact the other interfaces just convert data into mblk_t's and pass it down to the lower layer.
Work by Robert Mustacchi