net/smc: terminate link group without holding lgr lock

When a link group is to be terminated, it is sufficient to hold
the lgr lock when unlinking the link group from its list.
Move the lock-protected link group unlinking into smc_lgr_terminate().

Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>