pjdfstest: Fix link count test

[des]

Summary

This UFS-only test verifies that attempting to create more links than
permitted by the file system returns EMLINK, but has been broken ever
since UFS_LINK_MAX was increased because a) it hardcodes the previous
value of UFS_LINK_MAX, and b) the new value requires more space than
the test allocates, so it ends up getting ENOSPC instead of EMLINK.

• Switch to retrieving {PC_LINK_MAX} at runtime.

• Stop the test when we reach {PC_LINK_MAX} links. This ensures that we don't go on for hours if the actual limit turns out to be much higher than we anticipated (e.g. INT64_MAX on ZFS).

• Double the size of the test filesystem.

MFC after: 3 days
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Fixes: 35a301555bff ("Increase UFS/FFS maximum link count from 32767 to 65530.")

Test Plan

make -C /usr/src/tests/sys/pjdfstest all install
md=$(mdconfig -t malloc -s 32m)
newfs /dev/$md
mount /dev/$md /mnt
cd /mnt
/usr/tests/sys/pjdfstest/link/05

[des]

more idiomatic