mm: add kernel_move_pages() helper, move compat syscall to mm/migrate.c

mm: add kernel_move_pages() helper, move compat syscall to mm/migrate.c

Move compat_sys_move_pages() to mm/migrate.c and make it call a newly
introduced helper -- kernel_move_pages() -- instead of the syscall.

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527[email protected]

Cc: Al Viro <[email protected]>
Cc: [email protected]
Cc: Andrew Morton <[email protected]>
Signed-off-by: Dominik Brodowski <[email protected]>