vboxsf: Use flexible arrays for trailing string member

The declaration of struct shfl_string used trailing fake flexible arrays
for the string member. This was tripping FORTIFY_SOURCE since commit
df8fc4e9

 ("kbuild: Enable -fstrict-flex-arrays=3"). Replace the
utf8 and utf16 members with actual flexible arrays, drop the unused ucs2
member, and retriain a 2 byte padding to keep the structure size the same.

Reported-by: Larry Finger <Larry.Finger@lwfinger.net>
Closes: https://lore.kernel.org/lkml/ab3a70e9-60ed-0f13-e3d4-8866eaccc8c1@lwfinger.net/
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20230720151458.never.673-kees@kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>