erofs: clean up z_erofs_pcluster_readmore()

erofs: clean up z_erofs_pcluster_readmore()

`end` parameter is no needed since it's pointless for !backmost, we can
handle it with backmost internally.  And we only expand the trailing
edge, so the newstart can be replaced with ->headoffset.

Also, remove linux/prefetch.h inclusion since that is not used anymore
after commit 386292919c25 ("erofs: introduce readmore decompression
strategy").

Signed-off-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230525072605.17857-1-zbestahu@gmail.com
[ Gao Xiang: update commit description. ]
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>