xfs: t_firstblock is tracking AGs not blocks

xfs: t_firstblock is tracking AGs not blocks

Source kernel commit: 692b6cddeb65a5170c1e63d25b1ffb7822e80f7d

The tp->t_firstblock field is now raelly tracking the highest AG we
have locked, not the block number of the highest allocation we've
made. It's purpose is to prevent AGF locking deadlocks, so rename it
to "highest AG" and simplify the implementation to just track the
agno rather than a fsbno.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>