nfsd: allow nfsd_file_get to sanely handle a NULL pointer

...and remove some now-useless NULL pointer checks in its callers.
Suggested-by: NeilBrown <neilb@suse.de>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/filecache.c

@@ -452,7 +452,7 @@ static bool nfsd_file_lru_remove(struct nfsd_file *nf)
 struct nfsd_file *
 nfsd_file_get(struct nfsd_file *nf)
 {
-	if (likely(refcount_inc_not_zero(&nf->nf_ref)))
+	if (nf && refcount_inc_not_zero(&nf->nf_ref))
 		return nf;
 	return NULL;
 }
@@ -1107,8 +1107,7 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
 	rcu_read_lock();
 	nf = rhashtable_lookup(&nfsd_file_rhash_tbl, &key,
 			       nfsd_file_rhash_params);
-	if (nf)
-		nf = nfsd_file_get(nf);
+	nf = nfsd_file_get(nf);
 	rcu_read_unlock();
 
 	if (nf) {

fs/nfsd/nfs4state.c

@@ -602,9 +602,7 @@ put_nfs4_file(struct nfs4_file *fi)
 static struct nfsd_file *
 __nfs4_get_fd(struct nfs4_file *f, int oflag)
 {
-	if (f->fi_fds[oflag])
-		return nfsd_file_get(f->fi_fds[oflag]);
-	return NULL;
+	return nfsd_file_get(f->fi_fds[oflag]);
 }
 
 static struct nfsd_file *