ima: eliminate passing d_name.name to process_measurement()

Passing a pointer to the dentry name, as a parameter to
process_measurement(), causes a race condition with rename() and
is unnecessary, as the dentry name is already accessible via the
file parameter.

In the normal case, we use the full pathname as provided by
brpm->filename, bprm->interp, or ima_d_path().  Only on ima_d_path()
failure, do we fallback to using the d_name.name, which points
either to external memory or d_iname.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>