exportfs: make the "fsid=" parameter optional

Based on feedback [1] from the kernel developer @neilbrown regarding the
NFS clustering use case, it has been determined that the fsid= parameter
is now considered optional and safe to omit.

[1] https://bugzilla.suse.com/show_bug.cgi?id=1201271#c49
"""
Since some time in 2007 NFS has used the UUID of a filesystem as the
primary identifier for that filesystem, rather than using the device
number. So from that time there should have been reduced need for the
"fsid=" option. Probably there are some filesystems that this didn't
work for. btrfs has been problematic at time, particularly when subvols
are exported. But for quite some years this has all "just worked" at
least for the major filesystems (ext4 xfs btrfs). [...] I would suggest
getting rid of the use of fsid= altogether. [...] I'm confident that it
was no longer an issue in SLE-12 and similarly not in SLE-15.
"""