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