erofs: do not use pagepool in z_erofs_gbuf_growsize()

Let's use alloc_pages_bulk_array() for simplicity and get rid of
unnecessary pagepool.

Signed-off-by: Chunhai Guo <guochunhai@vivo.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240402092757.2635257-1-guochunhai@vivo.com


Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>