cachefiles: Fix double unlock in nomem_d_alloc error path

cachefiles: Fix double unlock in nomem_d_alloc error path

When start_creating() fails and returns -ENOMEM, it has already
released the parent directory lock in __start_dirop():

    static struct dentry *__start_dirop(...)
    {
        ...
        inode_lock_nested(dir, I_MUTEX_PARENT);
        dentry = lookup_one_qstr_excl(name, parent, lookup_flags);
        if (IS_ERR(dentry))
            inode_unlock(dir);  <-- Lock released on error
        return dentry;
    }

However, the nomem_d_alloc error path in cachefiles_get_directory()
unconditionally calls inode_unlock(d_inode(dir)) again, causing a
double unlock that corrupts the rwsem state.

This is a leftover from commit 7ab96df840e60 which replaced manual
locking with start_creating() but failed to update the nomem_d_alloc
path (while correctly updating mkdir_error and lookup_error paths).

Fixes: 7ab96df840e6 ("VFS/nfsd/cachefiles/ovl: add start_creating() and end_creating()")
Signed-off-by: Hongling Zeng <zenghongling@kylinos.cn>
Link: https://patch.msgid.link/20260617085049.730789-1-zenghongling@kylinos.cn
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>