CVE-2026-90139 fuse: check for NULL root inode in fuse_fill_super_submount

In the Linux kernel, the following vulnerability has been resolved: fuse: check for NULL root inode in fusefillsupersubmount fuseiget can return NULL when its inode allocation fails, but fusefillsupersubmount passed the result straight to getfuseinode and decremented fi-nlookup without checking i...