e4defrag: handle device symlinks

e4defrag: handle device symlinks

Device nodes are commonly accessed via symlinks, i.e.

lrwxrwxrwx. 1 root root 7 Jul 19 13:01 /dev/mapper/testvg-testlv -> ../dm-0

Today, e4defrag on such a device will fail:

File is not regular file
 "/dev/mapper/testvg-testlv"

due to it being a link, and e4defrag on the link target does as well:

Filesystem is not mounted

due to the target not being found in /etc/mtab.

Fix this by checking whether the symlink target is a block device
and if so, using that device in main(), and also changing get_mount_point()
to search for a matching device number, not device name.

Addresses-Red-Hat-Bugzilla: #707209

Reported-by: Peter Hjalmarsson <xake@rymdraket.net>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>