pipe: Fix iteration end check in fuse_dev_splice_write()

Fix the iteration end check in fuse_dev_splice_write().  The iterator
position can only be compared with == or != since wrappage may be involved.

Fixes: 8cefc107 ("pipe: Use head and tail pointers for the ring, not cursor and length")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>