xfs_syncd_start and xfs_syncd_stop tie a bunch of unrelated
functionailty together that actually have different start and stop
requirements. Kill these functions and open code the start/stop
methods for each of the background functions.

Subsequent patches will move the start/stop functions around to the
correct places to avoid races and shutdown issues.
Signed-off-by: NDave Chinner <dchinner@redhat.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NMark Tinguely <tinguely@sgi.com>
Signed-off-by: NBen Myers <bpm@sgi.com>