MIPS: crash_dump.c: Simplify copy_oldmem_page()

Replace kmap_atomic_pfn() with kmap_local_pfn() which is preemptible and
can take page faults.

Remove the indirection of the dump page and the related cruft which is not
longer required.

Remove unused or redundant header files.

Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>