vfs: clarify that nonseekable_open() will never fail

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: John Kacur <jkacur@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>