xfs: expose the blockgc workqueue knobs publicly

Expose the workqueue sysfs knobs for the speculative preallocation gc
workers on all kernels, and update the sysadmin information.

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

Documentation/admin-guide/xfs.rst

@@ -518,6 +518,9 @@ and the short name of the data device.  They all can be found in:
 ================  ===========
   xfs_iwalk-$pid  Inode scans of the entire filesystem. Currently limited to
                   mount time quotacheck.
+  xfs-blockgc     Background garbage collection of disk space that have been
+                  speculatively allocated beyond EOF or for staging copy on
+                  write operations.
 ================  ===========
 
 For example, the knobs for the quotacheck workqueue for /dev/nvme0n1 would be

fs/xfs/xfs_super.c

@@ -520,7 +520,7 @@ xfs_init_mount_workqueues(
 		goto out_destroy_cil;
 
 	mp->m_blockgc_workqueue = alloc_workqueue("xfs-blockgc/%s",
-			XFS_WQFLAGS(WQ_UNBOUND | WQ_FREEZABLE | WQ_MEM_RECLAIM),
+			WQ_SYSFS | WQ_UNBOUND | WQ_FREEZABLE | WQ_MEM_RECLAIM,
 			0, mp->m_super->s_id);
 	if (!mp->m_blockgc_workqueue)
 		goto out_destroy_reclaim;