nfs server: improve use of the VFS KPI

In particular, do not assume that vn_start_write() returns the same mp
as it was passed in, or never returns error.

Also be more accurate to return NULL vp and mp when error occured, to
catch wrong control flow easier.

Stop checking for NULL mp before calling vn_finished_write(), NULL mp
is handled transparently by the function.

Reviewed by: rmacklem
Tested by: pho
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27881