erofs: teach z_erofs_scan_folios() to handle multi-page folios

Previously, a folio just contains one page.  In order to enable large
folios, z_erofs_scan_folios() needs to handle multi-page folios.

First, this patch eliminates all gotos.  Instead, the new loop deal
with multiple parts in each folio.  It's simple to handle the parts
which belong to unmapped extents or fragment extents; but for encoded
extents, the page boundaries needs to be considered for `tight` and
`split` to keep inplace I/Os work correctly: when a part crosses the
page boundary, they needs to be reseted properly.

Besides, simplify `tight` derivation since Z_EROFS_PCLUSTER_HOOKED
has been removed for quite a while.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240703120051.3653452-3-hsiangkao@linux.alibaba.com