ANDROID: GKI: mount.h: add Android ABI padding to some structures

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

Based on a patch from Michal Marek <mmarek@suse.cz> from the SLES kernel

Bug: 151154716
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I9ce1b63f05c90af168eeea1312ac88d3cc5cfdf3