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: NMark Bloch <markb@mellanox.com>
Signed-off-by: NMaor Gottlieb <maorg@mellanox.com>
Signed-off-by: NLeon Romanovsky <leon@kernel.org>
Signed-off-by: NDoug Ledford <dledford@redhat.com>