RPCSEC_GSS: fix leak in krb5 code caused by superfluous kmalloc

I was sloppy when generating a previous patch; I modified the callers of
krb5_make_checksum() to allocate memory for the buffer where the result is
returned, then forgot to modify krb5_make_checksum to stop allocating that
memory itself.  The result is a per-packet memory leak.  This fixes the
problem by removing the now-superfluous kmalloc().

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>