nfsd: fix NFS regression

Fix regression introduced in pre-3.14 kernels by cherry-picking
aa07c713 (NFSD: Call ->set_acl with a
NULL ACL structure if no entries). This is in v3.12.22 as commit
723ac81c.

The affected code was removed in 3.14 by commit
4ac7249e (nfsd: use get_acl
and ->set_acl). The ->set_acl methods are already able to cope with a
NULL argument. So this is not needed for >= 3.14.
Signed-off-by: Sergio Gelato <Sergio.Gelato@astro.su.se>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

fs/nfsd/vfs.c

@@ -509,6 +509,9 @@ set_nfsv4_acl_one(struct dentry *dentry, struct posix_acl *pacl, char *key)
 	char *buf = NULL;
 	int error = 0;
 
+	if (!pacl)
+		return vfs_setxattr(dentry, key, NULL, 0, 0);
+
 	buflen = posix_acl_xattr_size(pacl->a_count);
 	buf = kmalloc(buflen, GFP_KERNEL);
 	error = -ENOMEM;