isofs compress: Remove VLA usage

As part of the effort to remove VLAs from the kernel[1], this changes
the allocation of the bhs and pages arrays from being on the stack to being
kcalloc()ed. This also allows for the removal of the explicit zeroing
of bhs.

https://lkml.org/lkml/2018/3/7/621



Signed-off-by: Kyle Spiers <ksspiers@google.com>
Signed-off-by: Jan Kara <jack@suse.cz>