Linux 2.6.26 compat, lookup_bdev()

It's doubtful many people were impacted by this but commit 6c28567
accidentally broke ZFS builds for 2.6.26 and earlier kernels.  This
commit depends on the lookup_bdev() function which exists in 2.6.26
but wasn't exported until 2.6.27.

The availability of the function isn't critical so a wrapper is
introduced which returns ERR_PTR(-ENOTSUP) when the function isn't
defined.  This will have the effect of causing zvol_is_zvol() to
always fail for 2.6.26 kernels.  This in turn means vdevs will
always get opened concurrently which is good for normal usage.
This will only become an issue if your using a zvol as a vdev in
another pool.  In which case you really should be using a newer
kernel anyway.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1205

include/linux/blkdev_compat.h

@@ -392,6 +392,15 @@ bio_set_flags_failfast(struct block_device *bdev, int *flags)
 # define vdev_bdev_invalidate(bdev)	invalidate_bdev(bdev, 1)
 #endif /* HAVE_1ARG_INVALIDATE_BDEV */
 
+/*
+ * 2.6.27 API change
+ * The function was exported for use, prior to this it existed by the
+ * symbol was not exported.
+ */
+#ifndef HAVE_LOOKUP_BDEV
+# define lookup_bdev(path)		ERR_PTR(-ENOTSUP)
+#endif
+
 /*
  * 2.6.30 API change
  * To ensure good performance preferentially use the physical block size