CVE-2026-45960: hfsplus: return error when node already exists in hfs_bnode_create

In the Linux kernel, the following vulnerability has been resolved:

hfsplus: return error when node already exists in hfsbnodecreate

When hfsbnodecreate() finds that a node is already hashed (which should not happen in normal operation), it currently returns the existing node without incrementing its reference count. This causes a reference count inconsistency that leads to a kernel panic when the node is later freed in hfsbnodeput():

kernel BUG at fs/hfsplus/bnode.c:676! BUGON(!atomicread(&node->refcnt))

This scenario can occur when hfsbmapalloc() attempts to allocate a node that is already in use (e.g., when node 0's bitmap bit is incorrectly unset), or due to filesystem corruption.

Returning an existing node from a create path is not normal operation.

Fix this by returning ERRPTR(-EEXIST) instead of the node when it's already hashed. This properly signals the error condition to callers, which already check for ISERR() return values.