dir: check afs_dir_Create return code in afs_dir_MakeDir

dir: check afs_dir_Create return code in afs_dir_MakeDir

afs_dir_MakeDir() ignores the return code from afs_dir_Create() for the
'.' and '..' ("dot" and "dotdot") directories.  This has been the case
from the earliest implementation (MakeDir() calling Create()) in the
original IBM import.

Instead, check the return codes to prevent the possibility of creating
malformed directories.

Change-Id: I60179488429dfa9afe60c4862c5e42b41f1e0048
Reviewed-on: https://gerrit.openafs.org/13800
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>