btrfs: apply first key check for readahead when possible

Currently for tree block readahead we never pass a
btrfs_tree_parent_check with @has_first_key set.

Without @has_first_key set, btrfs will skip the following extra
checks:

- Header generation check
This is a minor one.

- Empty leaf/node checks
This is more serious, for certain trees like the csum tree, they are
allowed to be empty, thus an empty leaf can pass the tree checker.
But if there is a parent node for such an empty leaf, it indicates
corruption.

Without @has_first_key set, we can no longer detect such a problem.

In fact there is already a fuzzed image report that a corrupted csum
leaf which has zero nritems but still has a parent node can trigger
a BUG_ON() during csum deletion.

However there are only two call sites of btrfs_readahead_tree_block():

- Inside relocate_tree_blocks()
At this call site we are trying to grab the first key of the tree
block, thus we are not able to pass a @first_key parameter.

- Inside btrfs_readahead_node_child()
This is the more common call site, where we have the parent node and
want to readahead the child tree blocks.

In this case we can easily grab the node key and pass it for checks.

Add a new parameter @first_key to btrfs_readahead_tree_block() and pass
the node key to it inside btrfs_readahead_node_child().

This should plug the gap in empty leaf detection during readahead.

Link: https://lore.kernel.org/linux-btrfs/20260409071255.3358044-1-gality369@gmail.com/
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>