Bluetooth: SMP: Fix SMP context init when sending security request

The code was not doing the right thing when we as peripheral would
send a security request to the central. First of all, the SEQ_REQ flag
was getting cleared by the pairing request handler, resulting in
pairing_confirm() callbacks for no reason. Secondly, the behavior in
encrypt_change() was not utilizing the smp_reset() helper as it should
have done.

Fix the situation by calling smp_init() when sending a security
request, and detect that this has been done when receiving a pairing
request. Also do the appropriate cleanup if the result is an encrypt
change instead of a pairing request (in case we were already paired
with the peer).

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>