ocfs2: Wrap calculation of name+value pair size.

An ocfs2 xattr entry stores the text name and value as a pair in the
storage area.  Obviously names and values can be variable-sized.  If a
value is too large for the entry storage, a tree root is stored instead.
The name+value pair is also padded.

Because of this, there are a million places in the code that do:

	if (needs_external_tree(value_size)
		namevalue_size = pad(name_size) + tree_root_size;
	else
		namevalue_size = pad(name_size) + pad(value_size);

Let's create some convenience functions to make the code more readable.
There are three forms.  The first takes the raw sizes.  The second takes
an ocfs2_xattr_info structure.  The third takes an existing
ocfs2_xattr_entry.
Signed-off-by: Joel Becker <joel.becker@oracle.com>