Increase devd's client socket buffer size

[asomers]

Summary

Increase devd's client socket buffer size to 256KB. This is not as large as it
looks, because we'll hit the sockbuf's mbuf limit long before hitting its data
limit. A 256KB data limit allows creating a ZFS pool on about 450 drives
without overflowing the client socket buffers.

Test Plan

Run existing ATF tests. Create ZFS pool on more than 250 drives and verify
that devd clients don't get HUPped.