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: I2766763d86048f026eeef0e0388b7de0e25c2093
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>