gpu: ion: Fix bug in ion_system_heap map_user

When the requested mmap length was not an integer number of
chunks or the buffer, or if an offset was provided, a bug
would cause extra or incorrect pages of the buffer to be mapped.

Change-Id: I254db0cc127475ca3b1ccf60f3f39792a339dff7
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
Git-commit: 319553d18cbda1a39a1723b6d9675e4a96829f11
Git-repo: https://android.googlesource.com/kernel/common


Signed-off-by: Laura Abbott <lauraa@codeaurora.org>