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.

Fixes: 284db12c ("staging: erofs: add trace points for reading zipped data")
Reviewed-by: Chen Gong <gongchen4@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>