erofs: refine readahead tracepoint

[ Upstream commit 4eb56b07 ]

 - trace_erofs_readpages => trace_erofs_readahead;

 - Rename a redundant statement `nrpages = readahead_count(rac);`;

 - Move the tracepoint to the beginning of z_erofs_readahead().

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Hongbo Li <lihongbo22@huawei.com>
Link: https://lore.kernel.org/r/20250514120820.2739288-1-hsiangkao@linux.alibaba.com


Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Stable-dep-of: d53238b6 ("erofs: fix to add missing tracepoint in erofs_readahead()")
Signed-off-by: Sasha Levin <sashal@kernel.org>