LU-11541 build: Adjust OFED check for DKMS enabled MLNX OFED.

LU-11541 build: Adjust OFED check for DKMS enabled MLNX OFED.

The DKMS packaging of MLNX OFED, i.e. mlnx-ofed-kernel-dkms, (at least
v4.4 and newer), doesn't have /usr/src/ofa_kernel/default as part of
the package, it is created as a link during package installation.

Restore LB_USES_DPKG autoconf test that was removed since it is needed
in lustre-lnet.m4 so Debian / Ubuntu can build properly with external
infiniband stacks.

Test-Parameters: trivial
Signed-off-by: Ake Sandgren <ake.sandgren@hpc2n.umu.se>
Change-Id: I1739a67e16bc0dab8cf551b460b424441d81fc91
Reviewed-on: https://review.whamcloud.com/33396
Tested-by: Jenkins
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>