innodb_log_spin_wait_delay_update(): Always acquire log_sys.latch
to protect the change of mtr_t::spin_wait_delay.

log_t::lock_lsn(): In the general case, actually use
mtr_t::spin_wait_delay as it was intended. In the x86 specific
log_t::lock_lsn_bts() we used mtr_t::spin_wait_delay.