cuse(3): Allow shared memory allocations up to, but excluding 2 GBytes.

Currently the cuse(3) mmap(2) offset is split into 128 banks of 16 Mbytes.
Allow cuse(3) to make allocations that span multiple banks at the expense
of any fragmentation issues that may arise. Typically mmap(2) buffers are
well below 16 Mbytes. This allows 8K video resolution to work using webcamd.

Reviewed by: markj @
Differential Revision: https://reviews.freebsd.org/D35830
MFC after: 1 week
Sponsored by: NVIDIA Networking