f2fs: fix calculating incorrect free size when update xattr in __f2fs_setxattr

During xattr updating, free size should be corrected to remainder free size
+ old entry size.
It can avoid ENOSPC error when we update old entry with the same size new
entry at fully filled xattr.
Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/xattr.c

@@ -520,7 +520,7 @@ static int __f2fs_setxattr(struct inode *inode, int name_index,
 		 */
 		free = MIN_OFFSET(inode) - ((char *)last - (char *)base_addr);
 		if (found)
-			free = free - ENTRY_SIZE(here);
+			free = free + ENTRY_SIZE(here);
 
 		if (free < newsize) {
 			error = -ENOSPC;