IB/core: Avoid unsigned int overflow in sg_alloc_table

BugLink: http://bugs.launchpad.net/bugs/1645453

commit 3c7ba576 upstream.

sg_alloc_table gets unsigned int as parameter while the driver
returns it as size_t. Check npages isn't greater than maximum
unsigned int.

Fixes: eeb8461e ("IB: Refactor umem to use linear SG table")
Signed-off-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/infiniband/core/umem.c

@@ -175,7 +175,7 @@ struct ib_umem *ib_umem_get(struct ib_ucontext *context, unsigned long addr,
 
 	cur_base = addr & PAGE_MASK;
 
-	if (npages == 0) {
+	if (npages == 0 || npages > UINT_MAX) {
 		ret = -EINVAL;
 		goto out;
 	}