Use dget_parent to handle getting inode of parent

Use dget_parent to handle getting inode of parent

We can't safely do dp->d_parent->d_inode to access the parent inode
of a given dentry. Instead, use dget_parent() to get safely get a
reference on the parent dcache, and access the inode using that
reference. Dispose of this reference with dput() as appropriate.

Change-Id: Ic4f2159f68f95744bca08ebc99ecd5f45dc46966
Reviewed-on: http://gerrit.openafs.org/1075
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>