btrfs: allow device add if balance is paused

Currently paused balance precludes adding a device since they are both
considered exclusive ops and we can have at most one running at a time.
This is problematic in case a filesystem encounters an ENOSPC situation
while balance is running, in this case the only thing the user can do
is mount the fs with "skip_balance" which pauses balance and delete some
data to free up space for balance. However, it should be possible to add
a new device when balance is paused.

Fix this by allowing device add to proceed when balance is paused.

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