Allow LVM to use USB Block devices (ub) for physical volumes.

Pete Zaitcev discovered that ub was not being considered for PVs, per
http://zaitcev.livejournal.com/183071.html

Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>

==
Take 3: 8 partitions only per zaitcev:
http://zaitcev.livejournal.com/183071.html?thread=354079

diff -Nuar LVM2.2.02.42.orig/lib/filters/filter.c LVM2.2.02.42/lib/filters/filter.c
--- LVM2.2.02.42.orig/lib/filters/filter.c	2008-04-03 03:29:00.000000000 -0700
+++ LVM2.2.02.42/lib/filters/filter.c	2009-01-20 12:08:08.364392500 -0800
@@ -78,6 +78,7 @@
 	{"vdisk", 8},		/* SUN's LDOM virtual block device */
 	{"ps3disk", 16},	/* PlayStation 3 internal disk */
 	{"virtblk", 8},		/* VirtIO disk */
+	{"ub", 8},			/* USB Block */
 	{NULL, 0}
 };