IB/mlx5: avoid bogus -Wmaybe-uninitialized warning

We get a false-positive warning in linux-next for the mlx5 driver:

infiniband/hw/mlx5/mr.c: In function ‘mlx5_ib_reg_user_mr’:
infiniband/hw/mlx5/mr.c:1172:5: error: ‘order’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
infiniband/hw/mlx5/mr.c:1161:6: note: ‘order’ was declared here
infiniband/hw/mlx5/mr.c:1173:6: error: ‘ncont’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
infiniband/hw/mlx5/mr.c:1160:6: note: ‘ncont’ was declared here
infiniband/hw/mlx5/mr.c:1173:6: error: ‘page_shift’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
infiniband/hw/mlx5/mr.c:1158:6: note: ‘page_shift’ was declared here
infiniband/hw/mlx5/mr.c:1143:13: error: ‘npages’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
infiniband/hw/mlx5/mr.c:1159:6: note: ‘npages’ was declared here

I had a trivial workaround for gcc-5 or higher, but that didn't work
on gcc-4.9 unfortunately.

The only way I found to avoid the warnings for gcc-4.9, short of
initializing each of the arguments first was to change the calling
conventions to separate the error code from the umem pointer. This
avoids casting the error codes from one pointer to another incompatible
pointer, and lets gcc figure out when that the data is actually valid
whenever we return successfully.

Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Doug Ledford <dledford@redhat.com>