fs: add generic FS_IOC_SHUTDOWN definitions

Currently, several filesystems (e.g., xfs, ext4, btrfs) implement
a "shutdown" or "going down" ioctl to simulate filesystem force a shutdown.
While they often use the same underlying numeric value, the definition is
duplicated across filesystem headers or private definitions.

Add generic definitions for FS_IOC_SHUTDOWN in uapi/linux/fs.h.
This allows new filesystems (like ntfs) to implement this feature using
a standard VFS definition and paves the way for existing filesystems
to unify their definitions later.

The flag names are standardized as FS_SHUTDOWN_* to be consistent with
the ioctl name, replacing the historical GOING_DOWN naming convention.

Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>