seq_file: disallow extremely large seq buffer allocations

There is no reasonable need for a buffer larger than this, and it avoids
int overflow pitfalls.

Fixes: 058504ed

 ("fs/seq_file: fallback to vmalloc allocation")
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Reported-by: Qualys Security Advisory <qsa@qualys.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>