mm: mm_struct: remove 16 bytes of alignment padding on 64 bit builds

Reorder mm_struct to remove 16 bytes of alignment padding on 64 bit
builds.  On my config this shrinks mm_struct by enough to fit in one
fewer cache lines and allows more objects per slab in mm_struct
kmem_cache under SLUB.

slabinfo before patch :-
    Sizes (bytes)     Slabs
    --------------------------------
    Object :     848  Total  :       9
    SlabObj:     896  Full   :       2
    SlabSiz:   16384  Partial:       5
    Loss   :      48  CpuSlab:       2
    Align  :      64  Objects:      18

 slabinfo after :-
    Sizes (bytes)     Slabs
    --------------------------------
    Object :     832  Total  :       7
    SlabObj:     832  Full   :       2
    SlabSiz:   16384  Partial:       3
    Loss   :       0  CpuSlab:       2
    Align  :      64  Objects:      19

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>