[do-not-merge] cherry pick multithreading support from mainline fuser

[dmreiter]

experimenting with write limits on kernel 5.15 if we pull in these changes from mainline fuser. motivated by: https://app.notion.com/p/modal-com/Volume-Benchmarking-3a61e7f16949801c88b0d5ce98acc8ce?source=copy_link#3b31e7f16949800380d7fb709c6bad6c

hitting ~40Gbps on writes with kernel 5.15, vs. ~20Gbps before. I had to turn off the write-back cache though, which also results in write serialization regardless of this fuser change.

[modal@ip-10-117-40-94 ~]$ ./fuse-benchmark --suite baseline --max-write 1M --block-size 1M --mounts 1 --jobs 1 2 4 8 16 32 --n-threads 8 --no-writeback-cache
suite,op,jobs,block_size,mib_per_s
...
baseline,write,8,1048576,4822.5
...
[modal@ip-10-117-40-94 ~]$ uname -a
Linux ip-10-117-40-94.ec2.internal 5.15.0-320.202.8.4.el9uek.x86_64 #2 SMP Mon May 18 19:18:45 PDT 2026 x86_64 x86_64 x86_64 GNU/Linux