nfsd: Don't set eof on a truncated READ_PLUS

If the READ_PLUS operation was truncated due to an error, then ensure we
clear the 'eof' flag.

Fixes: 9f0b5792 ("NFSD: Encode a full READ_PLUS reply")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>