Bluetooth: Never deallocate a session when some DLC points to it

Fix a bug introduced in commit 9cf5b0ea:
function rfcomm_recv_ua calls rfcomm_session_put without checking that
the session is not referenced by some DLC. If the session is freed, that
DLC would refer to deallocated memory, causing an oops later, as shown
in this bug report: https://bugzilla.kernel.org/show_bug.cgi?id=15994



Signed-off-by: Lukas Turek <8an@praha12.net>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>