Btrfs: fix reada debug code compilation

This fixes the following errors:

  fs/btrfs/reada.c: In function ‘btrfs_reada_wait’:
  fs/btrfs/reada.c:958:42: error: invalid operands to binary < (have ‘atomic_t’ and ‘int’)
  fs/btrfs/reada.c:961:41: error: invalid operands to binary < (have ‘atomic_t’ and ‘int’)

Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Cc: Chris Mason <chris.mason@fusionio.com>
Cc: linux-btrfs@vger.kernel.org
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>