btrfs: ioctl: remove unnecessary call to btrfs_mark_buffer_dirty()

btrfs: ioctl: remove unnecessary call to btrfs_mark_buffer_dirty()

The call to btrfs_mark_buffer_dirty() at btrfs_ioctl_default_subvol() is
not necessary as we have a path setup for writing with btrfs_search_slot()
having a 'cow' argument set to 1.

This just makes the code more verbose, confusing and add a little extra
overhead and well as increase the module's text size, so remove it.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>