splice: clear FMODE_NOWAIT on file if splice/vmsplice is used

In preparation for pipes setting FMODE_NOWAIT on pipes to indicate that
RWF_NOWAIT/IOCB_NOWAIT is fully supported, have splice and vmsplice
clear that file flag. Splice holds the pipe lock around IO and cannot
easily be refactored to avoid that, as splice and pipes are inherently
tied together.

By clearing FMODE_NOWAIT if splice is being used on a pipe, other users
of the pipe will know that the pipe is no longer safe for RWF_NOWAIT
and friends.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>