xfstests: add a helper to get the minimum dio size

Various tests opencode checks to find out the minimum support direct I/O
size.  Replace those with a generic helper that handles network filesystems as
well.  Also remove the Linux 2.4 workaround we had in once place.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>

tests/generic/240

@@ -60,7 +60,7 @@ rm -f $seqres.full
 
 rm -f $TEST_DIR/aiodio_sparse
 
-logical_block_size=`blockdev --getss $TEST_DEV`
+logical_block_size=`_min_dio_alignment $TEST_DEV`
 fs_block_size=`stat -f $TEST_DIR | grep "Block size:" | awk '{print $3}'`
 
 if [ $fs_block_size -le $logical_block_size ]; then