exofs: avoid VLA in structures

On the quest to remove all VLAs from the kernel[1] this adjusts several
cases where allocation is made after an array of structures that points
back into the allocation.  The allocations are changed to perform explicit
calculations instead of using a Variable Length Array in a structure.
Additionally, this lets Clang compile this code now, since Clang does not
support VLAIS[2].

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
[2] https://lkml.kernel.org/r/CA+55aFy6h1c3_rP_bXFedsTXzwW+9Q9MfJaW7GUmMBrAp-fJ9A@mail.gmail.com

Link: http://lkml.kernel.org/r/20180327203904.GA1151@beast


Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Cc: Boaz Harrosh <ooo@electrozaur.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>