cuse(3): Optimise small reads and writes.

When doing small reads and writes use an intermediate buffer to store the
data to save locking the remote process to access data.

Reviewed by: imp @
Sponsored by: NVIDIA Networking
Differential Revision: https://reviews.freebsd.org/D36633

(cherry picked from commit 5e59b2734f77da24e2bc45154c7db949b9d790c5)