The following 3 patches are changes to growfs necessary for supporting CRC
verification of metadata. The first converts AG btree root blocks to use the
generic btree header block initialisation code so CRC support only needs to be
added in one place. The second allows verifiers to distinguish between a write
for growfs whenteh xfs-perag is not fully initialised by using uncached buffers.
And the last adds initialisation of the AGFL during growfs, which we currently
don't do at all.
These are also patches for the 3.8 cycle i'd lik eto see in the 3.8 cycle - the
more infrastructure changes that get done in 3.8, the less there will be when
the format changes and CRCs patches arrive for the 3.9 cycle...