splice: fix i_mutex locking in generic_splice_write()

Rearrange locking of i_mutex on destination so it's only held while
buffers are copied with the pipe_to_file() actor, and not while
waiting for more data on the pipe.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>