xfs: consolidate the eofblocks and cowblocks workers

Remove the separate cowblocks work items and knob so that we can control
and run everything from a single blockgc work queue.  Note that the
speculative_prealloc_lifetime sysfs knob retains its historical name
even though the functions move to prefix xfs_blockgc_*.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>