Compress pending socket buffer data once it is marked ready.

Apply similar logic from sbcompress to pending data in the socket
buffer once it is marked ready via sbready. Normally sbcompress
merges small mbufs to reduce the length of mbuf chains in the socket
buffer. However, sbcompress cannot do this for mbufs marked
M_NOTREADY. sbcompress_ready is now called from sbready when mbufs
are marked ready to merge small mbuf chains once the data is available
to copy.

Submitted by: gallatin (earlier version)
Reviewed by: gallatin, hselasky, rrs
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D20616