btrfs: add a BTRFS_FS_NEED_TRANS_COMMIT flag

Currently we are only using fs_info->pending_changes to indicate that we
need a transaction commit.  The original users for this were removed
years ago and we don't have more usage in sight, so this is the only
remaining reason to have this field.  Add a flag so we can remove this
code.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>