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/ext4/xattr_security.c

@@ -33,8 +33,6 @@ ext4_xattr_security_get(const struct xattr_handler *handler,
 			struct dentry *dentry, const char *name,
 			void *buffer, size_t size)
 {
-	if (strcmp(name, "") == 0)
-		return -EINVAL;
 	return ext4_xattr_get(d_inode(dentry), EXT4_XATTR_INDEX_SECURITY,
 			      name, buffer, size);
 }
@@ -44,8 +42,6 @@ ext4_xattr_security_set(const struct xattr_handler *handler,
 			struct dentry *dentry, const char *name,
 			const void *value, size_t size, int flags)
 {
-	if (strcmp(name, "") == 0)
-		return -EINVAL;
 	return ext4_xattr_set(d_inode(dentry), EXT4_XATTR_INDEX_SECURITY,
 			      name, value, size, flags);
 }