Fix quota handling in vfs_ixnas.c and fake_file.c

Pcaps with TN 11.2-U6 show SMB2 Get_Info requests for FileQuotaInformation hanging and not responding. This has been implicated as possible cause for several types of backup software failing to properly generate backups. Since we're handling all quotas through vfs_ixnas (because the default backend isn't aware of ZFS userspace quotas), have the default vfs set/get quota functions return -1 with errno ENOSYS. This matches behavior observed in Samba 4.7.

We need an extra check in the set_nt_quota function in ixnas to match the permissions checking in fake_file.c.