IB/iser: use sector_div instead of do_div

do_div is the wrong way to divide a sector_t, as it is less
efficient when sector_t is 32-bit wide. With the upcoming
do_div optimizations, the kernel starts warning about this:

drivers/infiniband/ulp/iser/iser_verbs.c:1296:4: note: in expansion of macro 'do_div'
include/asm-generic/div64.h:224:22: warning: passing argument 1 of '__div64_32' from incompatible pointer type

This changes the code to use sector_div instead, which always
produces optimal code.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>