vmm(4): Bump VM_MAX_MEMMAPS for vmgenid

As a short term solution for the problem reported by Shawn Webb re: r359950,
bump the maximum number of memmaps per VM. This structure is 40 bytes, and the
additional four (fixed array embedded in the struct vm) members increase the
size of struct vm by 3%.

(The vast majority of struct vm is the embedded struct vcpu array, which
accounts for 84% of the size -- over 4 kB.)

Reported by: Shawn Webb <shawn.webb AT hardenedbsd.org>
Reviewed by: grehan
X-MFC-With: r359950
Differential Revision: https://reviews.freebsd.org/D24507