LU-4416 osd: using set_nlink to calm down WARN_ON message

Invoke inc_nlink with nlink=0 will trigger a WARN_ON
message. Flag I_LINKABLE can avoid this problem in
latest upstream kernel. But there exist a case that
WARN_ON was invoked and I_LINKABLE hasn't defined.
RHEL7 does so. Then we just using set_nlink in the
nlink=0 case.

Signed-off-by: Yang Sheng <yang.sheng@intel.com>
Change-Id: I1e110872b359ebd7fa883506c5592f072f26c1dc
Reviewed-on: http://review.whamcloud.com/11571
Tested-by: Jenkins
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>