sparc: fix size argument to find_next_zero_bit()

iommu_alloc_ctx() finds a zero bit in iommu->ctx_bitmap.  It starts
searching from iommu->ctx_lowest_free to the end of the bitmap.
But the size argument to find_next_zero_bit() in iommu_alloc_ctx()
is wrong.  It should be the bitmap size, not the maximum size to
search from the offset argument.

Fortunately iommu->ctx_lowest_free is almost unused and it will not
be more than 1. So the bug wasted only 1-bit at the end of
iommu->ctx_bitmap.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: sparclinux@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>