btrfs: add a btrfs_get_dev_args_from_path helper

btrfs: add a btrfs_get_dev_args_from_path helper

[ Upstream commit 00357dc29dd0b09806b4b19a5778d70e43e7ead5 ]

We are going to want to populate our device lookup args outside of any
locks and then do the actual device lookup later, so add a helper to do
this work and make btrfs_find_device_by_devspec() use this helper for
now.

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