xfs_repair: fix rebuilding btree block less than minrecs

xfs_repair: fix rebuilding btree block less than minrecs

In production, we found that sometimes xfs_repair phase 5
rebuilds freespace node block with pointers less than minrecs
and if we trigger xfs_repair again it would report such
the following message:

bad btree nrecs (39, min=40, max=80) in btbno block 0/7882

The background is that xfs_repair starts to rebuild AGFL
after the freespace btree is settled in phase 5 so we may
need to leave necessary room in advance for each btree
leaves in order to avoid freespace btree split and then
result in AGFL rebuild fails. The old mathematics uses
ceil(num_extents / maxrecs) to decide the number of node
blocks. That would be fine without leaving extra space
since minrecs = maxrecs / 2 but if some slack was decreased
from maxrecs, the result would be larger than what is
expected and cause num_recs_pb less than minrecs, i.e:

num_extents = 79, adj_maxrecs = 80 - 2 (slack) = 78

so we'd get

num_blocks = ceil(79 / 78) = 2,
num_recs_pb = 79 / 2 = 39, which is less than
minrecs = 80 / 2 = 40

OTOH, btree bulk loading code behaves in a different way.
As in xfs_btree_bload_level_geometry it wrote

num_blocks = floor(num_extents / maxrecs)

which will never go below minrecs. And when it goes above
maxrecs, just increment num_blocks and recalculate so we
can get the reasonable results.

Later, btree bulk loader will replace the current repair code.
But we may still want to look for a backportable solution
for stable versions. Hence, keep the same logic to avoid
the freespace as well as rmap btree minrecs underflow for now.

Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Eric Sandeen <sandeen@sandeen.net>
Fixes: 9851fd79bfb1 ("repair: AGFL rebuild fails if btree split required")
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>