libmount: read utab always when read mtab from /proc

libmount: read utab always when read mtab from /proc

Now libmount reads utab only when mtab filename is no explicitly
specified, but for example:

 mnt_table_parse_mtab(tb, "/proc/self/mountinfo");

ignores utab because filename points to regular file. This is mistake,
we wnat to read utab always when we read mount table from kernel.

Reported-by: Martin Pitt <martin.pitt@ubuntu.com>
Signed-off-by: Karel Zak <kzak@redhat.com>