erofs: fix to add missing tracepoint in erofs_readahead()

erofs: fix to add missing tracepoint in erofs_readahead()

Commit 771c994ea51f ("erofs: convert all uncompressed cases to iomap")
converts to use iomap interface, it removed trace_erofs_readahead()
tracepoint in the meantime, let's add it back.

Fixes: 771c994ea51f ("erofs: convert all uncompressed cases to iomap")
Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250707084832.2725677-1-chao@kernel.org
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>