LU-3696 mdd: decref volatile object after creation

Drop the nlink count on a volatile file after it is created.
It is created with a single reference and then added to the PENDING
directory, which adds a second reference to the inode.  When it is
closed it is left with an nlink count=1 and not unlinked as it should
be.

Do not insert volatile files into the ChangeLog, since they are
intended to be temporary files that cannot be accessed after creation
and should not be synched to a backup filesystem.

If llapi_create_volatile_idx() is used on a non-Lustre filesystem then
it should also unlink the temporary filename after opening it, so that
the behaviour is the same.  Improve the comments for this function.

Minor cleanups of related functions found during investigation.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I4bd37395a8eff94458e62414a5a4e7206b76d32d
Reviewed-on: http://review.whamcloud.com/10179
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Aurelien Degremont <aurelien.degremont@cea.fr>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>