stand: set st_dev/st_ino in the loader's ZFS stat for veriexecHEADmain

The loader's ZFS implementation never set st_dev or st_ino in
zfs_dnode_stat(). With an uninitialized struct stat, veriexec's device
comparison in lib/libsecureboot/veopen.c read stack garbage and skipped
the matching manifest entry, failing with a spurious "no entry" on ZFS
root under UEFI Secure Boot.

Rather than zeroing the device (which would break veriexec's ability to
tell apart the same path on different datasets), populate st_dev and
st_ino with the same intrinsic identifiers the kernel uses:

- st_dev = the dataset's ds_fsid_guid (as the kernel does via
  dmu_objset_fsid_guid()/dsl_dataset_fsid_guid()), already read in
  zfs_mount_dataset() and now propagated through struct zfsmount.
- st_ino = the object number resolved in zfs_lookup(), propagated
  through struct file (the loader's equivalent of the kernel's z_id).

dev_t and ino_t are 64-bit on FreeBSD, so both are assigned directly
with no hashing. A memset() at the top of zfs_dnode_stat() zeroes the
remaining fields so they no longer hold stack garbage.

Tested on 16.0-CURRENT (amd64), ZFS-on-GELI root: rebuilt and re-signed
the EFI loader; the system boots under UEFI Secure Boot with
mac_veriexec active.

(boot1 segment was modified by kevans)

PR:		295935
Sponsored by:	Defenso

Reviewed-by: kevans
Pull-Request: https://github.com/freebsd/freebsd-src/pull/2271

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