ANDROID: GKI: reserve one u64 field in mm_struct for future backports

mm_struct is one of the structures which are very hard to modify without breaking ABI because it is allocated statically in init-mm.c and we
can't allocate a larger container. To handle future backports which
need to add to mm_struct, reserve a field there which can be used to
store a pointer to a structure containing more fields.

Bug: 308042511
Change-Id: Ibee450616242d1223416e3ce2b8ee95c6f42a0ff
Signed-off-by: Suren Baghdasaryan <surenb@google.com>