dir: Honor non-ENOENT lookup errors

dir: Honor non-ENOENT lookup errors

Currently, several places in src/dir/dir.c assume that any error from
a lower-level function (e.g. FindItem) means that the item we're
looking for does not exist in that directory. But if we encountered
some other error, that may not be the case; the directory blob may be
corrupt, we may have encountered some I/O error, etc.

To detect cases like this, return the actual error code from FindItem
&c, instead of always reporting ENOENT. For the code paths that are
actually specifically looking for if the target exists (in
afs_dir_Create), change our checks to specifically check for ENOENT,
and return any other error.

Do the same thing for a few similar callers in viced/afsfileprocs.c,
as well.

FIXES 134904

Reviewed-on: https://gerrit.openafs.org/13431
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 0b3bd1b7cdc88ba62c8cd540e8628faa84e33cf9)

Change-Id: Ia81ff85821c1987b97390a683f1d442ca70db41e
Reviewed-on: https://gerrit.openafs.org/13543
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>