Pull up following revision(s) (requested by christos in ticket #738):

sys/ufs/ffs/ffs_vfsops.c: revision 1.355

PR/52728: Izumi Tsutsui: "mount -u /dev/ /" triggers kernel panic

Simplify the control flow of the mount code and make sure that the
mountfrom argument can be converted to a block device in the update
case.

(martin)