rpc: gss: fix a kmap_atomic race in krb5 code

This code is never called from interrupt context; it's always run by either
a user thread or rpciod.  So KM_SKB_SUNRPC_DATA is inappropriate here.

Thanks to Aimé Le Rouzic for capturing an oops which showed the kernel
taking an interrupt while we were in this piece of code, resulting in a
nested kmap_atomic(.,KM_SKB_SUNRPC_DATA) call from
xdr_partial_copy_from_skb().
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

net/sunrpc/auth_gss/gss_krb5_wrap.c

@@ -57,9 +57,9 @@ gss_krb5_remove_padding(struct xdr_buf *buf, int blocksize)
 					>>PAGE_CACHE_SHIFT;
 		int offset = (buf->page_base + len - 1)
 					& (PAGE_CACHE_SIZE - 1);
-		ptr = kmap_atomic(buf->pages[last], KM_SKB_SUNRPC_DATA);
+		ptr = kmap_atomic(buf->pages[last], KM_USER0);
 		pad = *(ptr + offset);
-		kunmap_atomic(ptr, KM_SKB_SUNRPC_DATA);
+		kunmap_atomic(ptr, KM_USER0);
 		goto out;
 	} else
 		len -= buf->page_len;