nfsd: Fix overflow causing non-working mounts on 1 TB machines

Since commit 10a68cdf10 (nfsd: fix performance-limiting session
calculation) (Linux 5.1-rc1 and 4.19.31), shares from NFS servers with
1 TB of memory cannot be mounted anymore. The mount just hangs on the
client.

The gist of commit 10a68cdf10 is the change below.

-avail = clamp_t(int, avail, slotsize, avail/3);
+avail = clamp_t(int, avail, slotsize, total_avail/3);

Here are the macros.

#define min_t(type, x, y) __careful_cmp((type)(x), (type)(y), <)
#define clamp_t(type, val, lo, hi) min_t(type, max_t(type, val, lo), hi)

`total_avail` is 8,434,659,328 on the 1 TB machine. `clamp_t()` casts
the values to `int`, which for 32-bit integers can only hold values
−2,147,483,648 (−2^31) through 2,147,483,647 (2^31 − 1).

`avail` (in the function signature) is just 65536, so that no overflow
was happening. Before the commit the assignment would result in 21845,
and `num = 4`.

When using `total_avail`, it is causing the assignment to be
18446744072226137429 (printed as %lu), and `num` is then 4164608182.

My next guess is, that `nfsd_drc_mem_used` is then exceeded, and the
server thinks there is no memory available any more for this client.

Updating the arguments of `clamp_t()` and `min_t()` to `unsigned long`
fixes the issue.

Now, `avail = 65536` (before commit 10a68cdf10 `avail = 21845`), but
`num = 4` remains the same.

Fixes: c54f24e338ed (nfsd: fix performance-limiting session calculation)
Cc: [e-mail]
Signed-off-by: Paul Menzel <[e-mail]>
Signed-off-by: J. Bruce Fields <[e-mail]>

fs/nfsd/nfs4state.c

···
 	 * Never use more than a third of the remaining memory,
 	 * unless it's the only way to give this client a slot:
 	 */
-	avail = clamp_t(int, avail, slotsize, total_avail/3);
+	avail = clamp_t(unsigned long, avail, slotsize, total_avail/3);
 	num = min_t(int, num, avail / slotsize);
 	nfsd_drc_mem_used += num * slotsize;
 	spin_unlock(&nfsd_drc_lock);