MFC r249951

According to devctl(4), clients must read events whole; they may not piece them
together from multiple reads().  It's as if /dev/devctl is a datagram device
instead of a stream device.  However, devd's internal buffer was too small
(1025 bytes) to read an entire ereport.fs.zfs.checksum event (variable, up to
~1300 bytes).  This commit enlarges the buffer to 8k.

Approved by:	ken (mentor)