Revert behavior of 59eab109 on not-Linux

It turns out that short-circuiting the EFAULT behavior on a short read
breaks things on FreeBSD. So until there's a nicer solution, let's
just revert the behavior for not-Linux.

Reference:
https://reviews.freebsd.org/R10:70f51f0e474ffe1fb74cb427423a2fba3637544d

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12698