Increase UFS/FFS maximum link count from 32767 to 65530.

The link count for a UFS/FFS inode is stored in a signed 16-bit
integer. Thus the maximum link count has been 32767.

This limit has been recently hit by the poudriere build system when
doing a ports build as it needs one directory per port and the
number of ports recently passed 32767.

A long-term solution would be to use one of the spare 32-bit fields
in the inode to store the link count. However, the UFS1 format does
not have a spare and adding the spare in UFS2 would make it hard
to make it compatible when running on older kernels that use the
original link count field. So this patch uses the much simpler
approach of changing the existing link count field from a signed
16-bit value to an unsigned 16-bit value. It has the fewest lines
of code changes. The only thing that changes is the type in the
dinode and inode structures and the definition of UFS_LINK_MAX. It
has the added benefit that it works with both UFS1 and UFS2.

It allows easy backward compatibility. Indeed it is backward
compatibility that is the primary reason to go with this approach.
If a filesystem with the new organization is mounted on an older
kernel, it still needs to work. Thus if we move the new link count
to a new field, we still need to maintain the old link count as
best as possible even when running on a kernel that knows about the
larger link counts. And we would have to carry this overhead for
the indefinite future.

If we have a new link-count field, we will have to add a new
filesystem flag to indicate that we are running with larger link
counts. We will also need to add of one of the new-feature flags
to say that we have larger link counts. Older kernels clear the
new-feature flags that they do not know about, so when a filesystem
is used on an older kernel and then moved back to a newer one, the
newer one will know that the new link counts have not been maintained
and that it will be necessary to run a full fsck on the filesystem
to correct the link counts before it can be mounted.

With this change, older kernels will generally work with the bigger
counts. While it will not itself allow the link count to exceed
32767, it will have no problem working with inodes that have a link
count greater than 32767. Since it tests that i_nlink <= UFS_LINK_MAX,
counts that are bigger than 32767 will appear negative, so will
still pass the test. Of course, if they ever drop below 32767, they
will no longer be able to exceed 32767. The one issue is if the
link count ever exceeds 65535 then it will wrap to zero and the
older kernel will be none the wiser. But this corner case is likely
to be very rare since these kernels and the applications running
on them do not expect to be able to get link counts over 32767. And
over time, the use of new filesystems on older kernels will become
rarer and rarer.

Reported-by: Mark Millard running poudriere on the ports tree
Reviewed-by: kib, olce.freebsd_certner.fr
Tested-by: Peter Holm, Mark Millard
MFC-after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D42767