fs_mgr: fix typo from off64_t change.

I accidentally changed some _mode enums from unsigned int to off64_t
along with the rest changes.  They probably shouldn't have been
unsigned anyway though, so let's make them plain 'int' while we fix
the original typo.

Test: boot
Change-Id: I0c6c51a1a8170056b99087d249539eb79026cffe