Btrfs: move some common code into a subfunction

Some code to open block devices, to read the superblock and to
handle errors was repeated multiple times in 3 places, and the
following patch makes use of it as well. This code is now moved
into a subfunction.

Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>