generic/050: Consistently remove traling dot from umount output

We did not consistently remove trailing dot from umount output which
can presumably lead to false failures with particular versions of
util-linux. Make sure all umount output is properly filtered.

Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>