nfsclient: eliminate ncl_writebp()

Use plain bufwrite() instead. ncl_writebp() evolved to mostly repeat
bufwrite() code with some ommisions, most notably runningbufspace
accounting.

Reviewed by: imp, markj, rmacklem
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D43249