LU-12527 utils: Make lustre_user.h c++-legal

LU-12527 utils: Make lustre_user.h c++-legal

recent c++ compilers did not like some of the C idioms used in this
header:
- C++ checks the types of enums more forecfully than is done in C
- signed vs unsigned comparisons will generate a warning under g++
- "invalid suffix on literal" warning: Lustre is not trying to
  generate a new literal identifier.

Signed-off-by: Rob Latham <robl@mcs.anl.gov>
Change-Id: I6aa8ba18407c14e071a7e2943b5a1a3f5be27bac
Reviewed-on: https://review.whamcloud.com/35471
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>