mountpoint: return dev_t from dir_to_device

mountpoint: return dev_t from dir_to_device

The string returned from this function was never of much use other than
to stat the path when the user requested a major:minor pair beyond the
true/false exit. Save some processing and directly returning the dev_t
on success, and an impossible value on failure.

[kzak@redhat.com: - use 0 as dir_to_device() return value on failure]

Signed-off-by: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Karel Zak <kzak@redhat.com>