firmware: qemu_fw_cfg: fix kobject leak in probe error path

firmware: qemu_fw_cfg: fix kobject leak in probe error path

An initialised kobject must be freed using kobject_put() to avoid
leaking associated resources (e.g. the object name).

Commit fe3c60684377 ("firmware: Fix a reference count leak.") "fixed"
the leak in the first error path of the file registration helper but
left the second one unchanged. This "fix" would however result in a NULL
pointer dereference due to the release function also removing the never
added entry from the fw_cfg_entry_cache list. This has now been
addressed.

Fix the remaining kobject leak by restoring the common error path and
adding the missing kobject_put().

Fixes: 75f3e8e47f38 ("firmware: introduce sysfs driver for QEMU's fw_cfg device")
Cc: stable@vger.kernel.org      # 4.6
Cc: Gabriel Somlo <somlo@cmu.edu>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20211201132528.30025-3-johan@kernel.org
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>