staging: ion: simplify ion_page_pool_total()

ion_page_pool_total() returns the total number of pages in the pool.
Depending on the argument passed, it counts highmem pages in or not.
This commit simplifies the code lines for better readability.

Signed-off-by: Heesub Shin <heesub.shin@samsung.com>
Reviewed-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>