nfsd: Use write gathering only with NFSv2

NFSv3 and above can use unstable writes whenever they are sending more
than one write, rather than relying on the flaky write gathering
heuristics. More often than not, write gathering is currently getting it
wrong when the NFSv3 clients are sending a single write with FILE_SYNC
for efficiency reasons.

This patch turns off write gathering for NFSv3/v4, and ensures that
it only applies to the one case that can actually benefit: namely NFSv2.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>