[Bug 240121] Serial console can eat a lot of cpu with law baudrate

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=240121

            Bug ID: 240121
           Summary: Serial console can eat a lot of cpu with law baudrate
           Product: Base System
           Version: 11.2-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: nicolas.masse at stormshield.eu

Since https://svnweb.freebsd.org/base?view=revision&revision=317659, devices
connected through a serial device and using a low baudrate seems to eat a lot
more cpu than before.
This has been constated using a lcd screen with a baudrate of 2400.
Using pmc, I got this:
PMC: [CPU_CLK_UNHALTED.THREAD_P] Samples: 14 (100.0%) , 0 unresolved

%SAMP CALLTREE
 85.7 amd64_syscall at kernel sys_write kern_writev dofilewrite devfs_write_f
ttydev_write ttydisc_write tty_wait _cv_wait_sig __mtx_lock_sleep
lock_delay(85.7%) 
  7.1 _start at xxx <no relevent information  here>(7.1%) 
  7.1 _start at xxx <no relevent information  here>(7.1%) 

Reverting this commit made the call to lock_delay disappear and the
performances of my test system back to normal.

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