Reject SMP pairing SMP_BUSY error while host is in pairing with another device

Steps to reproduce: |-
1. Turn on BT
2. Iniatited LE SMP pairing with remote 1
3. Initiate pairing from remote2 to DUT

Expected result: DUT should reject the SMP pairing req from remote2 with SMP_BUSY error
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Observed result: NA. its new feature from bluetooth specification 6.0

Fix:  Reject SMP pairing SMP_BUSY error while host is in pairing with another device

Test: atest automated-test
bug: 381996349
Flag: EXEMPT bugfix

Change-Id: I3f07864a9258bc8457e4d5e3f37e4ca717f42aa0