btrfs: Stop using call_rcu for device freeing

btrfs_device structs are freed from RCU context since device iteration
is protected by RCU. Currently this is achieved by using call_rcu since
no blocking functions are called within btrfs_free_device. Future
refactoring of pending/pinned chunks will require calling sleeping
functions.

This patch is in preparation for these changes by simply switching from
RCU callbacks to explicit calls of synchronize_rcu and calling
btrfs_free_device directly. This is functionally equivalent, making sure
that there are no readers at that time.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>