fs/reiserfs/inode.c: replace 0 by NULL for pointers

Fix sparse warning:

  fs/reiserfs/inode.c:2769:19: warning: Using plain integer as NULL pointer

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>