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

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

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

Bug: 151154716
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I2c41281b191cd252db2ddc792699d869ce8d7a4d