Revert "Revert "mm/rmap: Fix anon_vma->degree ambiguity leading to double-reuse""

This reverts commit 4f35cec76058557d9eaec0d501d03c7657eb56b4 and does so
in an abi-safe way.

This is done by adding the new fields only to the end of the structure
and this structure is only passed around to other functions as a
pointer, the internal structure layout is only touched by the core
kernel, so adding it to the end is safe.

ABI differences manually updated:

Leaf changes summary: 1 artifact changed
Changed leaf types summary: 1 leaf type changed
Removed/Changed/Added functions summary: 0 Removed, 0 Changed, 0 Added function
Removed/Changed/Added variables summary: 0 Removed, 0 Changed, 0 Added variable

'struct anon_vma at rmap.h:29:1' changed:
  type size changed from 704 to 832 (in bits)
  2 data member insertions:
    'unsigned long int num_children', at offset 704 (in bits) at rmap.h:70:1
    'unsigned long int num_active_vmas', at offset 768 (in bits) at rmap.h:72:1
  761 impacted interfaces

Bug: 260678056
Bug: 253167854
Change-Id: Ib1d45625cbc2e0b21330ca3dc2aa7aff34666d31
Signed-off-by: Lee Jones <joneslee@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
(cherry picked from commit d3e1a50cba092fa9c56fc642ee74f360c4b40a17)