usb: gadget: f_mass_storage: Fix NULL dereference in fsg_add()

usb: gadget: f_mass_storage: Fix NULL dereference in fsg_add()

fsg_common_init() can fail when memory is low. In that case, it returns
PTR_ERR().
fsg_add() does not check for failure, and thus dereferences an invalid
fsg_common later, which crashes.

Verify if we receive an error from fsg_common_init() and handle it
gracefully.

Reported-by: Zixun LI <admin@hifiphile.com>
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Tested-by: Zixun LI <admin@hifiphile.com> # on SAM9X60
Link: https://lore.kernel.org/r/20250328-ums-gadget-leak-v1-3-3b677db99bde@baylibre.com
Signed-off-by: Mattijs Korpershoek <mkorpershoek@kernel.org>