groups: use flexible-array member in struct group_info

Replace zero-size array with flexible array member, as recommended by
the docs.

Link: https://lkml.kernel.org/r/155995eed35c3c1bdcc56e69d8997c8e4c46740a.1611620846.git.hubert.jasudowicz@gmail.com


Signed-off-by: Hubert Jasudowicz <hubert.jasudowicz@gmail.com>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Micah Morton <mortonm@chromium.org>
Cc: Gao Xiang <xiang@kernel.org>
Cc: Michael Kelley <mikelley@microsoft.com>
Cc: Thomas Cedeno <thomascedeno@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>