Use offset_in_page() macro instead of open-coding.

Link: http://lkml.kernel.org/r/4dbc77ccaaed98b183cf4dba58a4fa325fd65048.1492758503.git.geliangtang@gmail.com


Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Cc: Mark Fasheh <mfasheh@versity.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Joseph Qi <jiangqi903@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>