f2fs: introduce discard_map for f2fs_trim_fs

This patch adds a bitmap for discard issues from f2fs_trim_fs.
There-in rule is to issue discard commands only for invalidated blocks
after mount.
Once mount is done, f2fs_trim_fs trims out whole invalid area.
After ehn, it will not issue and discrads redundantly.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.h

@@ -163,6 +163,7 @@ struct seg_entry {
 	 */
 	unsigned short ckpt_valid_blocks;
 	unsigned char *ckpt_valid_map;
+	unsigned char *discard_map;
 	unsigned char type;		/* segment type like CURSEG_XXX_TYPE */
 	unsigned long long mtime;	/* modification time of the segment */
 };