staging: erofs: localize UNALLOCATED_CACHED_PAGE placeholder

In practice, in order to do cached decompression rather than reuse
them for in-place decompression and make full use of pages in
page_pool instead of allocating as much as possible, an unallocated
placeholder was introduce to mark all in compressed_pages[] and
they will be replaced at the time of submission.

Previously EROFS_UNALLOCATED_CACHED_PAGE was included in internal.h,
which is unnecessary since it's only internally used in decompression
subsystem, move it to unzip_vle.c and rename it to PAGE_UNALLOCATED.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>