dracut-install: fix relative paths in --kerneldir

dracut-install: fix relative paths in --kerneldir

kerneldirlen is used to modify absolute path returned by
kmod_module_get_path() while it is calculated on user-supplied
--kerneldir argument which can be a relative path.

Use kmod_get_dirname() to convert user-supplied path to the same format
as used by kmod_module_get_path().

This also allows to get rid of now useless strcmp checks that seem to
imply that /lib and /usr/lib are linked which is not always true.