btrfs: add a BTRFS_FS_ERROR helper

We have a few flags that are inconsistently used to describe the fs in
different states of failure.  As of 5963ffca ("btrfs: always abort
the transaction if we abort a trans handle") we will always set
BTRFS_FS_STATE_ERROR if we abort, so we don't have to check both ABORTED
and ERROR to see if things have gone wrong.  Add a helper to check
BTRFS_FS_STATE_ERROR and then convert all checkers of FS_STATE_ERROR to
use the helper.

The TRANS_ABORTED bit check was added in af722733

 ("Btrfs: clean up
resources during umount after trans is aborted") but is not actually
specific.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>