svcrpc: fix memory leak in gssp_accept_sec_context_upcall

Our UC-KLEE tool found a kernel memory leak of 512 bytes (on x86_64) for
each call to gssp_accept_sec_context_upcall()
(net/sunrpc/auth_gss/gss_rpc_upcall.c). Since it appears that this call
can be triggered by remote connections (at least, from a cursory a
glance at the call chain), it may be exploitable to cause kernel memory
exhaustion. We found the bug in kernel 3.16.3, but it appears to date
back to commit 9dfd87da (2013-08-20).

The gssp_accept_sec_context_upcall() function performs a pair of calls
to gssp_alloc_receive_pages() and gssp_free_receive_pages().  The first
allocates memory for arg->pages.  The second then frees the pages
pointed to by the arg->pages array, but not the array itself.

Reported-by: David A. Ramos <daramos@stanford.edu>
Fixes: 9dfd87da ("rpc: fix huge kmalloc's in gss-proxy”)
Signed-off-by: David A. Ramos <daramos@stanford.edu>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>