rc: run the zfs rc script before tmp

The tmp rc script has much the same problem that the var does: it wants
to test if /tmp is writable, and mount a tmpfs if it's not.  This means
that we actually want our zfs datasets mounted first, because we might
have a /tmp dataset that changes the story.

The ordering problem is particularly noticable with a r/o zfs root,
since the write test will fail and we'll mount a tmpfs that later gets
covered by our /tmp dataset.  If that /tmp dataset inherited readonly,
then we're still in trouble.

This also fixes `tmpmfs=yes`, which would again get covered by a zfs
dataset with the existing ordering.

Reviewed by:	des
Differential Revision:	https://reviews.freebsd.org/D54995

libexec/rc/rc.d/zfs

@@ -4,7 +4,7 @@
 
 # PROVIDE: zfs
 # REQUIRE: zfsbe
-# BEFORE: FILESYSTEMS var
+# BEFORE: FILESYSTEMS tmp var
 
 . /etc/rc.subr