dir: check afs_dir_MakeDir return code in DirSalvage

dir: check afs_dir_MakeDir return code in DirSalvage

Since the original IBM import, DirSalvage() has ignored the return code
from afs_dir_MakeDir() (f.k.a. MakeDir).  This has been safe because, as
the comment states, afs_dir_MakeDir returns no (non-zero) error code.

In preparation for a future commit, add a check for the return from
afs_dir_MakeDir and remove the comment.

Change-Id: Ibb259a7aaeeb21ef70a7794143a0dadb2a75725d
Reviewed-on: https://gerrit.openafs.org/13799
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>