Add balance command group under both 'btrfs' and 'btrfs filesystem'.
Preserve the old 'btrfs filesystem balance <path>' behaviour.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
The new infrastructure offloads checking number of arguments passed to a
command to individual command handlers. Fix them up accordingly.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Separate every command group into its own file (cmds_<group>.c) and
rearrange includes. Remove btrfs_cmds.c.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
