xfs: push the grant head when the log head moves forward

When the log fills up, we can get into the state where the
outstanding items in the CIL being committed and aggregated are
larger than the range that the reservation grant head tail pushing
will attempt to clean. This can result in the tail pushing range
being trimmed back to the the log head (l_last_sync_lsn) and so
may not actually move the push target at all.

When the iclogs associated with the CIL commit finally land, the
log head moves forward, and this removes the restriction on the AIL
push target. However, if we already have transactions sleeping on
the grant head, and there's nothing in the AIL still to flush from
the current push target, then nothing will move the tail of the log
and trigger a log reservation wakeup.

Hence the there is nothing that will trigger xlog_grant_push_ail()
to recalculate the AIL push target and start pushing on the AIL
again to write back the metadata objects that pin the tail of the
log and hence free up space and allow the transaction reservations
to be woken and make progress.

Hence we need to push on the grant head when we move the log head
forward, as this may be the only trigger we have that can move the
AIL push target forwards in this situation.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>