Use a helper function to clarify gang block size

Use a helper function to clarify gang block size

For gang blocks, `DVA_GET_ASIZE()` is the total space allocated for the
gang DVA including its children BP's.  The space allocated at each DVA's
vdev/offset is `vdev_psize_to_asize(vd, SPA_GANGBLOCKSIZE)`.

This commit makes this relationship more clear by using a helper
function, `vdev_gang_header_asize()`, for the space allocated at the
gang block's vdev/offset.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #11744