xfs_repair: try to fill the AGFL before we fix the freelist

xfs_repair: try to fill the AGFL before we fix the freelist

In commit 9851fd79bfb1, we added a slight amount of slack to the free
space btrees being reconstructed so that the initial fix_freelist call
(which is run against a totally empty AGFL) would never have to split
either free space btree in order to populate the free list.

The new btree bulk loading code in xfs_repair can re-create this
situation because it can set the slack values to zero if the filesystem
is very full.  However, these days repair has the infrastructure needed
to ensure that overestimations of the btree block counts end up on the
AGFL or get freed back into the filesystem at the end of phase 5.

Fix this problem by reserving extra blocks in the bnobt reservation, and
checking that there are enough overages in the bnobt/cntbt fakeroots to
populate the AGFL with the minimum number of blocks it needs to handle a
split in the bno/cnt/rmap btrees.

Note that we reserve blocks for the new bnobt/cntbt/AGFL at the very end
of the reservation steps in phase 5, so the extra allocation should not
cause repair to fail if it can't find blocks for btrees.

Fixes: 9851fd79bfb1 ("repair: AGFL rebuild fails if btree split required")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>