nfsd4: don't try to encode conflicting owner if low on space

I ran into this corner case in testing: in theory clients can provide
state owners up to 1024 bytes long.  In the sessions case there might be
a risk of this pushing us over the DRC slot size.

The conflicting owner isn't really that important, so let's humor a
client that provides a small maxresponsize_cached by allowing ourselves
to return without the conflicting owner instead of outright failing the
operation.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>