etcfs: fuse passthrough

[elee1766]

adds fuse passthrough support to etcfs https://docs.kernel.org/6.16/filesystems/fuse-passthrough.html

checks for ETCFS_ENABLE_PASSTHROUGH env var to be 1 or yes to enable. it will fail to enable if the kernel does not support it, even if the env var is set.

requires a kernel 6.9+ compiled with FUSE_CAP_PASSTHROUGH (enabled by default)

i think it should come disabled out of the box, in case of possible bugs. i think that the changes to the non-passthrough paths are non-invasive enough that this feels safe.

what does fuse passthrough do:

in "traditional" fuse, on m_open/m_create, when interacting with an existing underlying filesystem, we open the fd, and then when m_read/m_write fuse callback is called, we read/write from said fd. this comes with the unfortunate side effect of forcing user-kernel space copies and context switches.

instead, with fuse passthrough, we can use ioctl to register the fd we open in m_open/m_crate with the kernel and obtain a "backing_id". this "backing_id", when added to the fuse id, allows fuse to skip the call to m_read/m_write, and "passthrough" the call to read/write directly to the fd registered with the backing id, thereby circumventing the requirement to cross user-kernel barrier

the consequences for etcfs are as follows:

1. two extra ioctl syscalls, one to register the backing fd on open/create, and one to unregister it on close.
2. no calls to m_read/m_write when a proxied fd is read or written once open, by etcfs.

limited benchmarks on my machine come back with these results. take these with a grain of salt. it is some ai generated python slop that i used to make the benchmark. i get varying results every time i run it, but it is always positive at least on my machine so far.
(14900@5.8ghz, btrfs on nvme, debian 13 kernel 6.12.48)

## WRITE TIMES

| Workload | Native | Traditional | Passthrough | PT Speedup |
|----------|--------|-------------|-------------|------------|
| 25k × 4KB (tiny files)       |  0.310s |       5.805s |       4.660s |       1.25x |
| 10k × 64KB (small files)     |  0.238s |       2.878s |       1.841s |       1.56x |
| 1k × 1MB (medium files)      |  0.194s |       1.019s |       0.549s |       1.86x |
| 100 × 10MB (large files)     |  0.213s |       0.678s |       0.265s |       2.55x |

## READ TIMES

| Workload | Native | Traditional | Passthrough | PT Speedup |
|----------|--------|-------------|-------------|------------|
| 25k × 4KB (tiny files)       |  0.099s |       5.633s |       4.211s |       1.34x |
| 10k × 64KB (small files)     |  0.119s |       3.058s |       1.896s |       1.61x |
| 1k × 1MB (medium files)      |  0.123s |       0.831s |       0.349s |       2.38x |
| 100 × 10MB (large files)     |  0.130s |       0.389s |       0.155s |       2.51x |