LU-5963 nodemap: use proper hashing

LU-5963 nodemap: use proper hashing

don't hash a export pointer as string.
check a situation when we don't delete a export from nodemap
hash.

Signed-off-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Change-Id: Id53281078f165ce984abebc74992bde30fcc9f31
Reviewed-on: http://review.whamcloud.com/12881
Tested-by: Jenkins
Reviewed-by: Andrew Perepechko <andrew_perepechko@xyratex.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Kit Westneat <kit.westneat@gmail.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>