erofs: mechanically convert erofs_read_metabuf() to offsets

just lift the call of erofs_pos() into the callers; it will
collapse in most of them, but that's better done caller-by-caller.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/r/20240425195846.GC1031757@ZenIV


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