repair: pull the build_agi() call up out of the inode tree build

repair: pull the build_agi() call up out of the inode tree build

Pull the build_agi() call out of build_ino_tree() in phase 5. This
is to prepare for finobt support, in which build_agi() will require
context from multiple inode tree reconstructions (both the inode
allocation tree and free inode tree, when it exists).

Create the new 'agi_stat' structure to carry the requisite state
from the build_ino_tree() operation to build_agi().

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>