tmpfs: increase vfs.tmpfs.memory_percent to 100 as workaround

The changes to avoid letting tmpfs use all of memory + swap do not
work well with ZFS ARC. The ARC can grow quite large, and will shrink
when there is memory pressure, but tmpfs does not allow for that.
Pending investigation of the right way to handle this, change the
default value of the vfs.tmpfs.memory_percent sysctl to 100 as a
workaround. The sysctl can be set to 95 to get back to the previous
default.