linux fh_to_dentry can return err

linux fh_to_dentry can return err

handle e.g. ESTALE from  fh_to_dentry (in this case via
d_obtain_alias and fat_fh_to_dentry) so we print a meaningful
error when we lose.

Change-Id: If9f53776e0df4bfecf704943110a62b8c98684ce
Reviewed-on: http://gerrit.openafs.org/3028
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>