blockdev: Use GCC_FMT_ATTR (format checking)

Additional changes:

* Removed 'extern' from drive_add (avoids too long line).
* Removed 'extern' from other functions (makes declarations
  consistent with others in same header file).

Cc: Blue Swirl <blauwirbel@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
