btrfs: simplify fs_devices member access in btrfs_init_dev_replace_tgtdev

In btrfs_init_dev_replace_tgtdev() we dereference fs_info to get
fs_devices many times, instead save a point to the fs_devices.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>