LU-6038 osd-zfs: Avoid redefining KM_SLEEP

LU-6038 osd-zfs: Avoid redefining KM_SLEEP

Due to some long overdue memory management cleanup in the ZoL kmem
implementation the definition of KM_SLEEP has changed.  This change
was expected to be transparent to consumers but it causes issues
for Lustre because it explicitly redefines KM_SLEEP.  This was
originally done to avoid overriding the Linux slab interfaces.

This change implements a more portable fix.  Instead of preventing
the inclusion of the kmem.h header by setting the guard.  The
kmem_cache_* preprocessor macros are explictly undefined to make
the Linux interface available.

The related ZoL pull requests are as follows:

  https://github.com/zfsonlinux/spl/pull/414
  https://github.com/zfsonlinux/zfs/pull/2918

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Change-Id: Id1d19d7b4c440808b8b3fd042f687b10c1b869f3
Reviewed-on: http://review.whamcloud.com/13096
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Isaac Huang <he.huang@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>