driver core: get_dev_from_fwnode(): document potential race

driver core: get_dev_from_fwnode(): document potential race

Commit 9a4681a485ee ("driver core: Export get_dev_from_fwnode()") made
get_dev_from_fwnode() publicly available, but didn't document the
guarantees a caller must uphold:

get_dev_from_fwnode() obtains a reference count from the device pointer
stored in a struct fwnode_handle. While having its own reference count,
struct fwnode_handle does not keep a reference count of the device it
has a pointer to.

Consequently, a caller must guarantee that it is impossible that the
last device reference is dropped and the device is released concurrently
while calling get_dev_from_fwnode(), otherwise this is a potential UAF
and hence a bug.

Thus, document this potential race condition for get_dev_from_fwnode().

Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Saravana Kannan <saravanak@google.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250829205911.33142-1-dakr@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>