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: NEric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>