nfsd: Fix ACL null pointer deref

BugLink: http://bugs.launchpad.net/bugs/1348670

Fix regression introduced in pre-3.14 kernels by cherry-picking
aa07c713
(NFSD: Call ->set_acl with a NULL ACL structure if no entries).

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.
Signed-off-by: Sergio Gelato <Sergio.Gelato@astro.su.se>
[bwh: Rewrite the subject]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/nfsd/vfs.c

@@ -508,6 +508,9 @@ 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;