uapi/fcntl: add FD_INVALID

Add a marker for an invalid file descriptor.

Link: https://lore.kernel.org/20250624-work-pidfs-fhandle-v2-7-d02a04858fe3@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

include/uapi/linux/fcntl.h

@@ -110,6 +110,7 @@
 #define PIDFD_SELF_THREAD		-10000 /* Current thread. */
 #define PIDFD_SELF_THREAD_GROUP		-10001 /* Current thread group leader. */
 
+#define FD_INVALID			-10009 /* Invalid file descriptor: -10000 - EBADF = -10009 */
 
 /* Generic flags for the *at(2) family of syscalls. */