btrfs: add a sanity check for btrfs root in btrfs_search_slot()

Syzbot reports a null-ptr-deref in btrfs_search_slot().

The reproducer is using rescue=ibadroots, and the extent tree root is
corrupted thus the extent tree is NULL.

When scrub tries to search the extent tree to gather the needed extent
info, btrfs_search_slot() doesn't check if the target root is NULL or
not, resulting the null-ptr-deref.

Add sanity check for btrfs root before using it in btrfs_search_slot().

Reported-by: syzbot+3030e17bd57a73d39bd7@syzkaller.appspotmail.com
Fixes: 42437a6 ("btrfs: introduce mount option rescue=ignorebadroots")
Link: https://syzkaller.appspot.com/bug?extid=3030e17bd57a73d39bd7
CC: stable@vger.kernel.org # 5.15+
Reviewed-by: Qu Wenruo <wqu@suse.com>
Tested-by: syzbot+3030e17bd57a73d39bd7@syzkaller.appspotmail.com
Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -2046,7 +2046,7 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
 		      const struct btrfs_key *key, struct btrfs_path *p,
 		      int ins_len, int cow)
 {
-	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct btrfs_fs_info *fs_info;
 	struct extent_buffer *b;
 	int slot;
 	int ret;
@@ -2059,6 +2059,10 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
 	int min_write_lock_level;
 	int prev_cmp;
 
+	if (!root)
+		return -EINVAL;
+
+	fs_info = root->fs_info;
 	might_sleep();
 
 	lowest_level = p->lowest_level;