video: tegra: nvmap: Fix overflow in nvmap_heap

do_heap_alloc locates a suitable free block from a nvmap heap given a
size and alignment. Unfortunately, if a heap block happens to be
smaller than the alignment passed to the function, an integer overflow
will occur, and a block that's too small gets accidentally returned.

Bug 1032642

Change-Id: Ic650c520409134d753e968f62f144ddeb065ccc7
Signed-off-by: Tuomas Tynkkynen <ttynkkynen@nvidia.com>
Reviewed-on: http://git-master/r/123076
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit
Reviewed-by: Krishna Reddy <vdumpa@nvidia.com>