LU-5396: define __must_hold() for older kernels

LU-5396: define __must_hold() for older kernels

Backport of sparse macro __must_hold(), introduced in linux kernel
commit 8529091e:

linux/compiler.h has macros to denote functions that acquire or release
locks, but not to denote functions called with a lock held that return
with the lock still held.  Add a __must_hold macro to cover that case.

Change-Id: Ic77304a5f78f1681cfc48a728a12759366bb2cb8
Signed-off-by: frank zago <fzago@cray.com>
Reviewed-on: http://review.whamcloud.com/11294
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>