btrfs: remove constant parameter to memset_extent_buffer and rename it

The only memset we do is to 0, so sink the parameter to the function and
simplify all calls. Rename the function to reflect the behaviour.

Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 24f04d7..a26202e 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -501,7 +501,7 @@ static noinline int create_subvol(struct inode *dir,
 		goto fail;
 	}
 
-	memset_extent_buffer(leaf, 0, 0, sizeof(struct btrfs_header));
+	memzero_extent_buffer(leaf, 0, sizeof(struct btrfs_header));
 	btrfs_set_header_bytenr(leaf, leaf->start);
 	btrfs_set_header_generation(leaf, trans->transid);
 	btrfs_set_header_backref_rev(leaf, BTRFS_MIXED_BACKREF_REV);