Cleanup bdev.c after splitting into modules

Cleanup bdev.c after splitting into modules

The function

	- bdev_get();

becomes static. It is called from nowhere else so far and never appeared in any
header.

Minor changes

	- Avoid comparisons between int and size_t types. Use size_t where
	  possible else cast to size_t when it makes sense.
	- insert missing spaces between operators
	- put declarations for all static functions at the top

Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>