e2scrub: check to make sure lvm2 is installed

e2scrub: check to make sure lvm2 is installed

Not all systems will have the lvm2 package installed, so check for
that.  Pretty much all systems should have util-linux installed, but
check for that as well.

Of course, if lvm2 is installed we shouldn't find any LVM devices ---
but eventually the Demon Murphy will find a way to make it happen. :-)

Also, set the PATH so we don't have to worry about the script failing
due to /sbin not being in the path.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>