Btrfs: use the device_list_mutex during write_dev_supers

write_dev_supers was changed to use RCU to protect the list of
devices, but it was then sleeping while it actually wrote the supers.
This fixes it to just use the mutex, since we really don't any
concurrency in write_dev_supers anyway.

Signed-off-by: Chris Mason <chris.mason@oracle.com>