btrfs: set fs_devices->seed directly

This is in preparation to move a section of code in __btrfs_open_devices()
into a new function so that it can be reused. As we set seeding if any of
the device is having SB flag BTRFS_SUPER_FLAG_SEEDING, so do it in the
device list loop itself. No functional changes.

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