losetup: handle symlinks in /dev/loop/

losetup: handle symlinks in /dev/loop/

The loop_scandir() expects all the files in /dev/loop/ to be actual
devices and not symlinks to devices.  However, udev by default sets up
symlinks in /dev/loop/ thereby breaking the fallback scan logic when
looking for devices more than the default number of 8.

Simple way to reproduce is:
 - <detach all devices>
 - cd /dev/loop
 - rm -f [2-7]
 - losetup 0 autogen.sh
 - mv 1 8
 - for n in {1..7} ; do ln -s 0 $n ; done
 - losetup -f
	<error but should have found /dev/loop/8>

[kay.sievers@vrfy.org:
       just a note, udev does not do that by default. It is
       devfs-nonsense, which the "default" never did.]

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Reported-by: Mario Bachmann <grafgrimm77@gmx.de>