vfs: Distinguish between full xattr names and proper prefixes

Add an additional "name" field to struct xattr_handler.  When the name
is set, the handler matches attributes with exactly that name.  When the
prefix is set instead, the handler matches attributes with the given
prefix and with a non-empty suffix.

This patch should avoid bugs like the one fixed in commit c361016a in
the future.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: James Morris <james.l.morris@oracle.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/jffs2/security.c

@@ -52,9 +52,6 @@ static int jffs2_security_getxattr(const struct xattr_handler *handler,
 				   struct dentry *dentry, const char *name,
 				   void *buffer, size_t size)
 {
-	if (!strcmp(name, ""))
-		return -EINVAL;
-
 	return do_jffs2_getxattr(d_inode(dentry), JFFS2_XPREFIX_SECURITY,
 				 name, buffer, size);
 }
@@ -63,9 +60,6 @@ static int jffs2_security_setxattr(const struct xattr_handler *handler,
 				   struct dentry *dentry, const char *name,
 				   const void *buffer, size_t size, int flags)
 {
-	if (!strcmp(name, ""))
-		return -EINVAL;
-
 	return do_jffs2_setxattr(d_inode(dentry), JFFS2_XPREFIX_SECURITY,
 				 name, buffer, size, flags);
 }