btrfs: switch all message helpers to be RCU safe

We have two versions of message helpers, one that provides RCU
protection around the call in case we need to dereference device name.
As messages are not performance critical we can set up the RCU
protection for all of them and drop the distinction for those where
device name is needed. This will lead to further simplifications.

Reviewed-by: Daniel Vacek <neelx@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>