Revert "nfsd4: remove check_conflicting_opens warning"

This reverts commit 50747dd5 "nfsd4: remove check_conflicting_opens
warning", as a prerequisite for reverting 94415b06

, which has a
serious bug.

Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>