btrfs: properly register fs backing device

btrfs assigns this bdi to all inodes on that file system, so make
sure it's registered. This isn't really important now, but will be
when we put dirty inodes there. Even now, we miss the stats when the
bdi isn't visible.

Also fixes failure to check bdi_init() return value, and bad inherit of
->capabilities flags from the default bdi.

Acked-by: Chris Mason <chris.mason@oracle.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>