nfs: Move call to security_inode_listsecurity into nfs_listxattr

Add a nfs_listxattr operation.  Move the call to security_inode_listsecurity
from list operation of the "security.*" xattr handler to nfs_listxattr.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: Anna Schumaker <anna.schumaker@netapp.com>
Cc: linux-nfs@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/xattr.c b/fs/xattr.c
index c3af6c9..2c77764 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -723,6 +723,8 @@
 
 	if (!buffer) {
 		for_each_xattr_handler(handlers, handler) {
+			if (!handler->list)
+				continue;
 			size += handler->list(handler, dentry, NULL, 0,
 					      NULL, 0);
 		}
@@ -730,6 +732,8 @@
 		char *buf = buffer;
 
 		for_each_xattr_handler(handlers, handler) {
+			if (!handler->list)
+				continue;
 			size = handler->list(handler, dentry, buf, buffer_size,
 					     NULL, 0);
 			if (size > buffer_size)