libmount: don't canonicalize target

libmount: don't canonicalize target

Note that mountpoint (target_ paths in /proc/mounts and /proc/self/mountinfo
are always canonicalized by kernel.

 * for umount we don't have to canonicalize target
   by default if the mountpoint is found in /proc/self/mountinfo

 * in mnt_table_find_target() is unnecessary to canonicalize target paths
   if the table of the filesystems is read from /proc/self/mountinfo

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=820707
Signed-off-by: Karel Zak <kzak@redhat.com>