riscv: mm: define copy_user_page() as copy_page()

riscv: mm: define copy_user_page() as copy_page()

Currently, the implementation of copy_user_page() is identical to
copy_page().

Align riscv with other architectures (alpha, arc, arm64, hexagon,
longarch, m68k, openrisc, s390, um, xtensa) and map copy_user_page()
to copy_page() given that their implementation is identical.

In addition to following a common pattern, this centralizes the
implementation. Any changes to the underlying page copy logic (e.g.,
for CHERI) will now automatically propagate to copy_user_page().

Signed-off-by: Florian Schmaus <florian.schmaus@codasip.com>
Link: https://patch.msgid.link/20260113134025.905627-1-florian.schmaus@codasip.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>