common: new function to get real device path name and basename

common: new function to get real device path name and basename

If TEST_DEV or SCRATCH_DEV is symlink(mostly a lvm lv), a simple
basename is not enough, symlink should be followed.

This task is common enough, so introduce new helper functions and
replace all readlink calls in

ext4/305
generic/009
generic/019
generic/285
generic/312

Signed-off-by: Eryu Guan <eguan@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>