KVM: arm64/sve: Simplify KVM_REG_ARM64_SVE_VLS array sizing

A complicated DIV_ROUND_UP() expression is currently written out
explicitly in multiple places in order to specify the size of the
bitmap exchanged with userspace to represent the value of the
KVM_REG_ARM64_SVE_VLS pseudo-register.

To make this more readable, this patch replaces these with a single
define.

Since the number of words in a bitmap is just the index of the last
word used + 1, this patch expresses the bound that way instead.
This should make it clearer what is being expressed.

Since use of DIV_ROUND_UP() was the only reason for including
<linux/kernel.h> in guest.c, this patch removes that #include.

No functional change.

Suggested-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>