ANDROID: GKI: mm.h: add Android ABI padding to a structure

Try to mitigate potential future driver core api changes by adding a
padding to struct vm_operations_struct.

Based on a change made to the RHEL/CENTOS 8 kernel.

Bug: 151154716
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I78f84148ef4d3524bd6c5b78e53e06503a4ac3ae