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

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: 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().

References

• https://git.kernel.org/stable/c/c71d114ef68c95da5a82ec85a721ab31f5bd905b
• https://git.kernel.org/stable/c/db66fb87c21e8ae724886e6a464dcbac562a64c6
• https://git.kernel.org/stable/c/757171d1369b3b47f36932d40a05a0715496dcab
• https://git.kernel.org/stable/c/93992c3d9629b02dccf6849238559d5c24f2dece
• https://git.kernel.org/stable/c/3ed51857a50f530ac7a1482e069dfbd1298558d4