USB: fix testing the wrong variable in fs_create_by_name()

There is a typo here.  We should be testing "*dentry" which was just
assigned instead of "dentry".  This could result in dereferencing an
ERR_PTR inside either usbfs_mkdir() or usbfs_create().

Signed-off-by: Dan Carpenter <error27@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>