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 hfs_bnode_create

When hfs_bnode_create() 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 hfs_bnode_put():

kernel BUG at fs/hfsplus/bnode.c:676!

BUG_ON(!atomic_read(&node->refcnt))

This scenario can occur when hfs_bmap_alloc() 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 ERR_PTR(-EEXIST) instead of the node when it's

already hashed. This properly signals the error condition to callers,

which already check for IS_ERR() return values.