gpu: ion: Fix bug in ion_system_heap map_user

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.

Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
[jstultz: modified patch to apply to staging directory]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>