retpoline: move lock/unlock of spec_ctrl_mutex to check_modinfo()

retpoline: move lock/unlock of spec_ctrl_mutex to check_modinfo()

Testing has found that check_modinfo() will call disable_retpoline()
concurrently for the same module.  Serialization through
spec_ctrl_mutex in disable_retpoline() keeps the retpoline state sane
but misleading messages are issued during the duplicate call.
Move the lock/unlock of spec_ctrl_mutex to check_modinfo() so that the
first call to disable_retpoline() will disable retpoline before the
duplicate call checks to see if retpoline is disabled.

Orabug: 27625404
Signed-off-by: Chuck Anderson <chuck.anderson@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>