lib/canonicalize: fix truncation warning

lib/canonicalize: fix truncation warning

lib/canonicalize.c: In function ‘canonicalize_dm_name’:
lib/canonicalize.c:42:45: warning: ‘%s’ directive output may be truncated
writing up to 255 bytes into a region of size 244 [-Wformat-truncation=]
   snprintf(path, sizeof(path), "/dev/mapper/%s", name);

Notice that this warnign fix does not improve code enormously.  The earlier
snprintf() truncation will not happen a bit earlier when fgets() is called.
In that sense this change merely makes one easy to silence warning to
disappear, and therefore improve change of noticing useful messaging as such
crops up.

[kzak@redhat.com: - use macro rather than hardcoded string for mapper path]

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
Signed-off-by: Karel Zak <kzak@redhat.com>