btrfs: use nodesize everywhere, kill leafsize

The nodesize and leafsize were never of different values. Unify the
usage and make nodesize the one. Cleanup the redundant checks and
helpers.

Shaves a few bytes from .text:

  text    data     bss     dec     hex filename
852418   24560   23112  900090   dbbfa btrfs.ko.before
851074   24584   23112  898770   db6d2 btrfs.ko.after

Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>
diff --git a/fs/btrfs/reada.c b/fs/btrfs/reada.c
index 20408c6..b63ae20 100644
--- a/fs/btrfs/reada.c
+++ b/fs/btrfs/reada.c
@@ -347,7 +347,7 @@
 	if (!re)
 		return NULL;
 
-	blocksize = btrfs_level_size(root, level);
+	blocksize = root->nodesize;
 	re->logical = logical;
 	re->blocksize = blocksize;
 	re->top = *top;