netdump: Buffer pages to avoid calling netdump_send() on each 4KB write.

netdump waits for acknowledgement from the server for each write. When
dumping page table pages, we perform many small writes, limiting
throughput. Use the netdump client's buffer to buffer small contiguous
writes before calling netdump_send() to flush the MAXDUMPPGS-sized
buffer. This results in a significant reduction in the time taken to
complete a netdump.

Submitted by: Sam Gwydir <sam@samgwydir.com>
Reviewed by: cem
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D20317