nfsd: fix BUG at fs/nfsd/nfsfh.h:199 on unlink

As of commit 43a9aa64 "NFSD:
Fill in WCC data for REMOVE, RMDIR, MKNOD, and MKDIR", we sometimes call
fh_unlock on a filehandle that isn't fully initialized.

We should fix up the callers, but as a quick fix it is also sufficient
just to remove this assertion.
Reported-by: NMarius Tolzmann <tolzmann@molgen.mpg.de>
Cc: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfsfh.h

@@ -196,8 +196,6 @@ fh_lock(struct svc_fh *fhp)
 static inline void
 fh_unlock(struct svc_fh *fhp)
 {
-	BUG_ON(!fhp->fh_dentry);
-
 	if (fhp->fh_locked) {
 		fill_post_wcc(fhp);
 		mutex_unlock(&fhp->fh_dentry->d_inode->i_mutex);