Bluetooth: Fix bt_skb_sendmmsg not allocating partial chunks

Since bt_skb_sendmmsg can be used with the likes of SOCK_STREAM it
shall return the partial chunks it could allocate instead of freeing
everything as otherwise it can cause problems like bellow.

Fixes: 81be03e0 ("Bluetooth: RFCOMM: Replace use of memcpy_from_msg with bt_skb_sendmmsg")
Reported-by: Paul Menzel <pmenzel@molgen.mpg.de>
Link: https://lore.kernel.org/r/d7206e12-1b99-c3be-84f4-df22af427ef5@molgen.mpg.de
BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=215594


Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Tested-by: Paul Menzel <pmenzel@molgen.mpg.de> (Nokia N9 (MeeGo/Harmattan)
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>