ANDROID: mm: page_pinner: fix build warning

Remove the build warning below.

mm/page_pinner.c:201:28: warning: comparison of distinct pointer types ('typeof ((ts_usec)) *' (aka 'long long *') and 'uint64_t *' (aka 'unsigned long long *')) [-Wcompare-distinct-pointer-types]
                unsigned long rem_usec = do_div(ts_usec, 1000000);

Bug: 218731671
Signed-off-by: Minchan Kim <minchan@google.com>
Change-Id: I4d7b24998c3288f4066b5f88d5ebbf59e04b9873