erofs: drop the old pagevec approach

Remove the old pagevec approach but keep z_erofs_page_type for now.
It will be reworked in the following commits as well.

Also rename Z_EROFS_NR_INLINE_PAGEVECS as Z_EROFS_INLINE_BVECS with
the new value 2 since it's actually enough to bootstrap.

Acked-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220715154203.48093-6-hsiangkao@linux.alibaba.com