btrfs: add __cold attribute to more functions

The attribute can mark functions supposed to be called rarely if at all
and the text can be moved to sections far from the other code. The
attribute has been added to several functions already, this patch is
based on hints given by gcc -Wsuggest-attribute=cold.

The net effect of this patch is decrease of btrfs.ko by 1000-1300,
depending on the config options.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>