bpf: fix crash in bpf_[set|remove]_dentry_xattr for negative dentries

In the Linux kernel, the following vulnerability has been resolved: bpf: fix crash in bpf_[set|remove]_dentry_xattr for negative dentries bpf_set_dentry_xattr and bpf_remove_dentry_xattr BPF kfuncs attempt to lock the inode of the supplied dentry without checking if it is NULL. If a negative dentry is passed (e.g. from security_inode_create), d_inode(dentry) returns NULL, and inode_lock(inode) will cause a NULL pointer dereference. Trivially fix this by adding a NULL check for inode before attempting to lock it, returning -EINVAL if it is NULL. Additionally, drop WARN_ON(!inode) in bpf_xattr_read_permission() and bpf_xattr_write_permission(). These warnings could be triggered by passing a negative dentry to bpf_get_dentry_xattr() or the _locked variants of the xattr kfuncs, potentially causing a Denial of Service on systems with panic_on_warn enabled. Instead, simply return -EINVAL.