xfs: xfs_iflock is no longer a completion

With the recent rework of the inode cluster flushing, we no longer
ever wait on the the inode flush "lock". It was never a lock in the
first place, just a completion to allow callers to wait for inode IO
to complete. We now never wait for flush completion as all inode
flushing is non-blocking. Hence we can get rid of all the iflock
infrastructure and instead just set and check a state flag.

Rename the XFS_IFLOCK flag to XFS_IFLUSHING, convert all the
xfs_iflock_nowait() test-and-set operations on that flag, and
replace all the xfs_ifunlock() calls to clear operations.

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>
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 71ac6c1..68ec8db 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -654,11 +654,11 @@ xfs_fs_destroy_inode(
 	ASSERT_ALWAYS(!xfs_iflags_test(ip, XFS_IRECLAIM));
 
 	/*
-	 * We always use background reclaim here because even if the
-	 * inode is clean, it still may be under IO and hence we have
-	 * to take the flush lock. The background reclaim path handles
-	 * this more efficiently than we can here, so simply let background
-	 * reclaim tear down all inodes.
+	 * We always use background reclaim here because even if the inode is
+	 * clean, it still may be under IO and hence we have wait for IO
+	 * completion to occur before we can reclaim the inode. The background
+	 * reclaim path handles this more efficiently than we can here, so
+	 * simply let background reclaim tear down all inodes.
 	 */
 	xfs_inode_set_reclaim_tag(ip);
 }