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>