depmod: rewrite depmod modules search with scratchbuf

depmod: rewrite depmod modules search with scratchbuf

The recursive search code used used pretty big, PATH_MAX,
automatic storage buffer for the module directory scanning. Some
time ago there was scratchbuf implemented, which dynamically
reallocates its buffer on demand. The patch takes it in use for
the scanning code also. The initial size is hardcoded to 256
bytes which sounds good enough for most usecases so there should
be not many reallocations.

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>