f2fs: add reclaiming control by sysfs

This patch adds a control method in sysfs to reclaim prefree segments.

Signed-off-by: Changman Lee <cm224.lee@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

Documentation/filesystems/f2fs.txt

@@ -164,6 +164,12 @@ Files in /sys/fs/f2fs/<devname>
                               gc_idle = 1 will select the Cost Benefit approach
                               & setting gc_idle = 2 will select the greedy aproach.
 
+ reclaim_segments             This parameter controls the number of prefree
+                              segments to be reclaimed. If the number of prefree
+			      segments is larger than this number, f2fs tries to
+			      conduct checkpoint to reclaim the prefree segments
+			      to free segments. By default, 100 segments, 200MB.
+
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 USAGE
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