find_mount_point: fix find_mount_point for char devices

find_mount_point: fix find_mount_point for char devices

This allows to find mount points of 'char' devices such as UBI volumes
which otherwise fail for example with 'df' command:

/ # df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/root             72091984  12360980  56068924  18% /
devtmpfs                115236         4    115232   0% /dev
tmpfs                    28672        32     28640   0% /tmp
/dev/ubi0_0             360268     18348    341920   5% /tmp/mnt/userfs

/ # df /dev/ubi0_0
Filesystem           1K-blocks      Used Available Use% Mounted on
devtmpfs                115236         4    115232   0% /dev

Signed-off-by: Javier Viguera <javier.viguera@digi.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>