LU-15417 build: find the new path for MOFED 5.5

LU-15417 build: find the new path for MOFED 5.5

The path of the mofed header files has change to
/usr/src/ofa_kernel/x86_64/<kernel>,
so we cannot assume it's /usr/src/ofa_kernel/default.

Besides updating lbuild, we also need to update
lustre-lnet.m4 and lustre.spec.in.

Test-Parameters: trivial

Change-Id: Iab42ce9e458f78b0dc0233ac6fd23a1760be5324
Fixes: 94a3f1bfa70 ("LU-15417 build: build MOFED 5.5")
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/46383
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>