zfs: Set f_fsid in zfs_statfs

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
vfs/zfs: Add st_dev field support to stat

Tomek says:

Some programs rely on the fact that the (stat.st_ino, stat.st_dev) pair
uniquely identifies the file.
In OSv only st_ino is set by vn_stat(), st_dev is zeroed. It is possible that
files on different file systems will have the same inode number and hence
appear as referring to the same file.

Spotted during code inspection of openssl and vfs, I didn't run into any issue
related to that yet.

This patch addresses the issue #216

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
tests: Check st_dev and f_fsid validity

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>