IB: Fix ib_umem_get() when npages == 0

gcc correctly warned:

drivers/infiniband/core/umem.c: In function 'ib_umem_get':
drivers/infiniband/core/umem.c:78: warning: 'ret' may be used uninitialized in this function

Set ret to 0 in case npages == 0 and the loop isn't entered at all.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Roland Dreier <rolandd@cisco.com>