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

Try to mitigate potential future driver core api changes by adding a
padding to a bunch of filesystem structures.

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

Bug: 151154716
Change-Id: Ida6d98d30f292c980ab07e0250fec5268c4c87ed
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>