glm.c: detect 9p via statfs f_type, not the /mnt/ path prefix

[tonuonu]

Problem

The slow-filesystem warning at model load fires for any model path under /mnt/:

if(!strncmp(snap,"/mnt/",5))
    fprintf(stderr,"WARNING: the model is on %s (slow 9p/Windows filesystem; fadvise is ineffective).\n"
                   "         Keep it on ext4 (for example, /home/...) ...", snap);

The intent is to catch WSL, where Windows drives (/mnt/c, …) are slow 9p mounts. But /mnt/ is also a perfectly normal mount point on native Linux — ZFS/ext4/xfs/NFS pools commonly live there. On such a box this is a false positive that tells users their fast local storage is slow.

Repro: put the model on a native-Linux mount under /mnt (e.g. a ZFS pool at /mnt/data) → WARNING: ... slow 9p/Windows filesystem ... even though it's fast local storage.

Fix

Check the actual filesystem type via statfs() against the 9p superblock magic (0x01021997) instead of guessing from the path. Linux-only (<sys/vfs.h>); no change on macOS/Windows. A real WSL /mnt/c (9p) still warns; native /mnt/* (ZFS 0x2fc12fc1, ext4 0xef53, xfs, NFS…) no longer does.

Test

Native-Linux box, model on a ZFS pool at /mnt/data:

• Before: WARNING: ... slow 9p/Windows filesystem ...
• After: no warning. statfs("/mnt/data").f_type == 0x2fc12fc1 (ZFS) ≠ 0x01021997 (9p). Builds clean (make glm).

+13 −3, single file.