staging: erofs: fix `trace_erofs_readpage' position
`trace_erofs_readpage' was designed for .readpage() interface hook
in order to trace the detailed synchronized read rather than
the internal implementation `z_erofs_do_read_page' which both
.readpage() and .readpages() use.
It seems the tracepoint was placed to a wrong place by mistake and
it should be moved to the right place.