lseek(fd, n, SEEK_END) does *not* go to eof - n

When you copy some code, you are supposed to read it.  If nothing else,
there's a chance to spot and fix an obvious bug instead of sharing it...

X-Song: "I Got It From Agnes", by Tom Lehrer
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[ Tom Lehrer? You're dating yourself, Al ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>