btrfs: Sector Size check during Mount

Gracefully fail when trying to mount a BTRFS file system that has a
sectorsize smaller than PAGE_SIZE.

On PPC it is possible to build a FS while using a 4k PAGE_SIZE kernel
then boot into a 64K PAGE_SIZE kernel.  Presently open_ctree fails in an
endless loop and hangs the machine in this situation.

My debugging has show this Sector size < Page size to be a non trivial
situation and a graceful exit from the situation would be nice for the
time being.
Signed-off-by: Keith Mannthey <kmannth@us.ibm.com>

fs/btrfs/disk-io.c

@@ -2258,6 +2258,12 @@ struct btrfs_root *open_ctree(struct super_block *sb,
 		goto fail_sb_buffer;
 	}
 
+	if (sectorsize < PAGE_SIZE) {
+		printk(KERN_WARNING "btrfs: Incompatible sector size "
+		       "found on %s\n", sb->s_id);
+		goto fail_sb_buffer;
+	}
+
 	mutex_lock(&fs_info->chunk_mutex);
 	ret = btrfs_read_sys_array(tree_root);
 	mutex_unlock(&fs_info->chunk_mutex);