xfs_repair: reserve per-AG space while rebuilding rt metadata

xfs_repair: reserve per-AG space while rebuilding rt metadata

Realtime metadata btrees can consume quite a bit of space on a full
filesystem.  Since the metadata are just regular files, we need to
make the per-AG reservations to avoid overfilling any of the AGs while
rebuilding metadata.  This avoids the situation where a filesystem comes
straight from repair and immediately trips over not having enough space
in an AG.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>