NFS: remove unused check for negative dentry

mainline inclusion
from mainline-v5.4-rc1
commit 581057c8
category: bugfix
bugzilla: NA
CVE: NA

--------------------------------

This check has been hanging out since we used to have parallel paths to add
dentry in nfs_create(), but that hasn't been the case for some years.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>