nfsd4: fix file open accounting for RDWR opens

Commit f9d7562f

 "nfsd4: share file
descriptors between stateid's" didn't correctly account for O_RDWR opens.
Symptoms include leaked files, resulting in failures to unmount and/or
warnings about orphaned inodes on reboot.

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