[MIPS] unexport copy_from_user_page()

No users for the copy_from_user_page() routine exist outside of the
core kernel code. Therefore, EXPORT_SYMBOL(copy_from_user_page) is
useless, and this patch removes it.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>