Another possible dereference detected by coverity (id #759).  pf_probe()
might call pf_identify() which might call get_capacity() which dereferences
pf->disk
Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>