This fixes endless loop without schedule which happens as soon as smbd
invokes fcntl64(7, F_SETLK64, ...).  fcntl_setlk64 gets cmd F_SETLK64,
not F_SETLK tested in the loop;

Maybe return value from posix_lock_file should be changed to -EINPROGRESS
or -EJUKEBOX instead of testing passed cmd in callers, but this oneliner
works too. If you preffer changing posix_lock_file return value to clearly
distinugish between -EAGAIN and lock request queued, I'll do that.