filedesc: Close race between fcntl(F_SETFL) and ioctl(FIONBIO/FIOASYNC)

• Use the recently-added fsetfl_lock/unlock API to synchronize direct FIONBIO and FIOASYNC ioctls with fcntl(F_SETFL).

• While here, skip calling the underlying ioctl if the flag's current state matches the requested state.

• Also while here, only update the flag state if the underlying ioctl succeeds. This fixes a bug where the flags represented the new state even if the underlying ioctl failed. A test is added for this last case that a failing FIOASYNC on /dev/null doesn't result in setting O_ASYNC in the file flags.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D52721