xfs: add background scanning to clear eofblocks inodes

Create a new mount workqueue and delayed_work to enable background
scanning and freeing of eofblocks inodes. The scanner kicks in once
speculative preallocation occurs and stops requeueing itself when
no eofblocks inodes exist.

The scan interval is based on the new
'speculative_prealloc_lifetime' tunable (default to 5m). The
background scanner performs unfiltered, best effort scans (which
skips inodes under lock contention or with a dirty cache mapping).

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
index a631ca3..dc306a0 100644
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h
@@ -196,6 +196,8 @@
 #endif
 	struct xfs_mru_cache	*m_filestream;  /* per-mount filestream data */
 	struct delayed_work	m_reclaim_work;	/* background inode reclaim */
+	struct delayed_work	m_eofblocks_work; /* background eof blocks
+						     trimming */
 	__int64_t		m_update_flags;	/* sb flags we need to update
 						   on the next remount,rw */
 	struct shrinker		m_inode_shrink;	/* inode reclaim shrinker */
@@ -207,6 +209,7 @@
 	struct workqueue_struct	*m_cil_workqueue;
 	struct workqueue_struct	*m_reclaim_workqueue;
 	struct workqueue_struct	*m_log_workqueue;
+	struct workqueue_struct *m_eofblocks_workqueue;
 } xfs_mount_t;
 
 /*