Use FUNCTIONFS_ENDPOINT_ALLOC_BUFFER ioctl to improve performance.

The ioctl preallocates kernel buffers for the read/write syscalls,
reducing memory fragmentation and allowing for larger buffers,
as well as removing the kmalloc overhead.

Test: Manual
Change-Id: I9f5fef1b1625c1a2cda374541d8042f7e0ba78f5