[$] Zero-copy for FUSE

In a combined storage and filesystem session at the 2025 Linux Storage,
Filesystem, Memory Management, and BPF Summit (LSFMM+BPF), Keith Busch led
a discussion about zero-copy operations for the Filesystem
in Userspace (FUSE) subsystem. The session was proposed
by his colleague, David Wei, who could not make it to the summit, so Busch
filled in, though he noted that 'I do not really know FUSE so
well'. The idea is to eliminate data copies in the data path to and
from the FUSE server in user space.