btrfs: add MODULE_DESCRIPTION()

Fix the 'make W=1' warning:
WARNING: modpost: missing MODULE_DESCRIPTION() in fs/btrfs/btrfs.o
Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/super.c

@@ -2581,6 +2581,7 @@ static int __init init_btrfs_fs(void)
 late_initcall(init_btrfs_fs);
 module_exit(exit_btrfs_fs)
 
+MODULE_DESCRIPTION("B-Tree File System (BTRFS)");
 MODULE_LICENSE("GPL");
 MODULE_SOFTDEP("pre: crc32c");
 MODULE_SOFTDEP("pre: xxhash64");