reiserfs: Deprecate reiserfs

Reiserfs is relatively old filesystem and its development has ceased
quite some years ago. Linux distributions moved away from it towards
other filesystems such as btrfs, xfs, or ext4. To reduce maintenance
burden on cross filesystem changes (such as new mount API, iomap, folios
...) let's add a deprecation notice when the filesystem is mounted and
schedule its removal to 2025.

Link: https://lore.kernel.org/r/20220225125445.29942-1-jack@suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>