We have to have a valid sbi here, or we'd have oopsed already.  (There's a
dereference of sbi->catatonic a few lines above)

Coverity #740
Signed-off-by: Dave Jones <davej@redhat.com>
Cc: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>