blocktest: resolve label/minor in userland

The test script now resolves the device node into a <label,minor>
pair, so that the blocktest driver itself no longer has to. This
removes blocktest's dependency on VFS' internal data structures.

Also allow blocktest to be linked using with gcc/clang.

test/blocktest/README

@@ -5,6 +5,3 @@ Instructions on how to use blocktest:
 3) edit and uncomment one line from test.sh
 4) run './test.sh'
 5) read the output in /var/log/messages
-
-WARNING: while this tool works with both VFS and AVFS, it has been configured
-to use AVFS's headers. Either use with AVFS, or edit blocktest.c first.