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.

MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D4476