btrfs: clean up transaction abort messages

The transaction abort stacktrace is printed only once per module
lifetime, but we'd like to see it each time it happens per filesystem.
Introduce a fs_state flag that records the state.

Tweak the messages around abort:
* add error number to the first abor
* print the exact negative errno from btrfs_decode_error and
  don't expect a simple snprintf to fail
* no dots at the end of the messages

Signed-off-by: David Sterba <dsterba@suse.cz>