MFC: r220519

MFC: r220519
Fix a bug in the userland rpc library, where it would use a
negative return value from write to update its position in
a buffer. The patch, courtesy of Andrey Simonenko, also simplifies
a conditional by removing the "i != cnt" clause, since it is
always true at this point in the code. The bug caused problems
for mountd, when it generated a large reply to an exports RPC
request.

git-svn-id: svn://svn.freebsd.org/base/stable/8@220999 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f